The Physics Book

ELECTRICITY AND MAGNETISM 149 See also: Magnetism 122–123 ■ Electric potential 128–129 ■ Electric current and resistance 130–133 ■ Making magnets 134–135 ■ The motor effect 136–137 ■ Induction and the generator effect 138–141 ■ Force fields and Maxwell’s equations 142–147 Thomas Edison installed six huge between magnetic and electric Gramme’s dynamo was the first dynamos in the Pearl Street Station fields (Faraday’s law, or the induction electric motor to enter production. plant in Manhattan, New York, in 1882. principle) and used this knowledge It became widely used in Each of the dynamos produced enough to build the first electric generator, manufacturing and farming. electricity to power 1,200 lights. or dynamo, in 1831. This consisted of a highly conductive copper disk Industry demanded ever more Italian scientist Alessandro rotating between the poles of a efficient manufacturing processes Volta converted chemical energy horseshoe magnet, whose magnetic to increase production. This was to electricity to produce a steady field produced an electric current. the age of prolific invention, and current in a battery (later known Edison was an outstanding as a Voltaic pile). Although this Faraday’s explanation of the example, turning ideas into was the first electric battery, principles of mechanical electricity commercial gold at his workshops it was impractical, unlike that built generation—and his dynamo— and laboratories. His “light bulb by British inventor John Daniell would become the basis for the moment” came when he devised in 1836. The component parts of a more powerful generators of a lighting system to replace gas Daniell cell were a copper pot filled the future, but in the early 19th lamps and candles in homes, with copper sulfate solution, century there wasn’t yet the factories, and public buildings. in which was immersed an demand for large voltages. Edison didn’t invent the light earthenware container filled with bulb, but his 1879 carbon- zinc sulfate and a zinc electrode. Electricity for telegraphy, filament incandescent design was Negatively charged ions migrated arc lighting, and electroplating economical, safe, and practical for to one electrode, and positively was supplied by batteries, but home use. It ran on low voltage but charged ions to the other, creating this process was very expensive, required a cheap, steady form of an electric current. and scientists from several nations electricity to make it work. sought alternatives. French inventor The first dynamo Hippolyte Pixii, Belgian electrical Jumbo dynamos In the 1820s, British physicist engineer Zenobe Gramme, and Edison’s power plants transformed Michael Faraday experimented German inventor Werner von mechanical energy into electricity. with magnets and coils of insulated Siemens all worked independently A boiler fired by burning coal wire. He discovered the relationship to develop Faraday’s induction converted water to high-pressure principle in order to generate steam within a Porter-Allen steam electricity more efficiently. In 1871, engine. The shaft of the engine was connected directly to the armature (rotating coil) of the ❯❯ We will make electricity so cheap that only the rich will burn candles. Thomas Edison

150 GENERATING ELECTRICITY Industry needs to increase production, and to do low voltage and high current had so needs more power. a limited range due to resistance in the wires. To get around this Direct current (DC) Generators produce problem, Edison proposed a system electricity is low voltage and high-voltage alternating of local power plants that would provide electricity for local cannot be carried far, so has current (AC). neighborhoods. Because of the limited use. problem transmitting over long distances, these generating Step-up transformers (near generators) increase voltage stations needed to be located to carry current long distances. within 1 mile (1.6 km) of the user. Edison had built 127 of these stations by 1887, but it was clear that large parts of the United States would not be covered even with thousands of power plants. AC electricity is Step-down transformers The rise of AC electricity carried huge distances reduce voltage for safe use Serbian-American electrical and powers industry. engineer Nikola Tesla had suggested in homes and industry. an alternative—using alternating dynamo for greater efficiency. current (AC) generators— in which Edison used six “Jumbo” dynamos incandescent light bulbs. About the polarity of the voltage in a coil in his plant at Pearl Street Station, 110 volts of direct current (DC) reverses as the opposite poles of a which were four times larger than power was carried underground in rotating magnet pass over it. This any ever previously built. Each copper wires, which ran through regularly reverses the direction of weighed 30 tons (27 metric tons), insulating tubes. current in the circuit; the faster produced 100 kilowatts of the magnet turns, the quicker the electricity, and could light 1,200 In the late 19th century, it wasn’t current flow reverses. When possible to convert DC to a high American electronics engineer voltage, and power carried at a William Stanley built the first workable transformer in 1886, the voltage of transmission from an Primary coil around Secondary coil has Power stations produce high-current, low-voltage iron core receives twice as many wire electricity, which then has to be boosted to a high voltage electricity supply turns, doubling (low current) so it can be carried more efficiently over long Power the voltage distances. Before it can be used in industry or homes, the station current passes through a step-down transformer, which converts it to a lower voltage. Iron core Low current Secondary coil High voltage has fewer wire turns, reducing Primary coil the voltage Step-up Step-down transformer transformer High current High current Low voltage Low voltage

ELECTRICITY AND MAGNETISM 151 The deadly electricity larger operating plants employed Thomas Edison of the alternating current higher steam pressures for greater can do no harm unless a man efficiency. During the 20th century, One of the most prolific is fool enough to swallow electricity generation increased inventors of all time, Edison almost 10-fold. High-voltage AC had more than 1,000 patents a whole dynamo. transmission enabled power to be to his name by the time of his George Westinghouse moved from distant power stations death. He was born in Milan, to industrial and urban centers Ohio, and was mostly home- AC generator could be “stepped up” hundreds or even thousands of taught. In an early sign of and the current simultaneously miles away. his entrepreneurial talent, he “stepped down” in smaller-diameter gained exclusive rights to sell wire. At higher voltages, the same At the start of the century, coal newspapers while working power could be transmitted at a was the primary fuel burned to as a telegraph operator with much lower current, and this high create the steam to drive turbines. Grand Trunk Railway. His first voltage could be transmitted over It was later supplemented by other invention was an electronic long distances, then “stepped fossil fuels (oil and natural gas), vote recorder. down” again before use in a wood chippings, and enriched factory or home. uranium in nuclear power stations. At the age of 29, Edison established an industrial American entrepreneur George Sustainable solutions research lab in New Jersey. Westinghouse realized the problems With fears about rising atmospheric Some of his most famous of Edison’s design, purchased CO2 levels, sustainable alternatives patents were for radical Tesla’s many patents, and hired to fossil fuels have been introduced. improvements to existing Stanley. In 1893, Westinghouse Hydroelectric power, which dates devices, such as the telephone, won a contract to build a dam and back to the 19thcentury, now microphone, and light bulb. hydroelectric power plant at Niagara produces almost one-fifth of the Others were revolutionary, Falls to harness the immense water world’s electricity. The power of including the phonograph in power to produce electricity. The wind, waves, and tides is now 1877. He went on to found the installation was soon transmitting harnessed to drive turbines and General Electric Company AC to power the businesses and provide electricity. Steam originating in 1892. Edison was a homes of Buffalo, New York, and deep in Earth’s crust produces vegetarian and proud that marked the beginning of the end geothermal energy in parts of the he never invented weapons. for DC as the default means of world such as Iceland. About 1 transmission in the United States. percent of global electricity is now Key work generated by solar panels. Scientists New, super-efficient steam are also working to develop 2005 The Papers of Thomas turbines allowed generating hydrogen fuel cells. ■ A. Edison: Electrifying capacity to expand rapidly. Ever- New York and Abroad, April 1881–March 1883 Solar panels are made of dozens of photovoltaic cells, which absorb solar radiation, exciting free electrons and generating a flow of electrical current.

152 IN CONTEXT NSCAAOTSTENMUPTARRILNEOLLTHOEF KEY FIGURE John Bardeen (1908–1991) ELECTRONICS BEFORE 1821 German physicist Thomas Seebeck observes a thermoelectric effect in a semiconductor. 1904 British engineer John Ambrose Fleming invents the vacuum tube diode. 1926 Austrian–American engineer Julius Lilienfeld patents the FET (field-effect transistor). 1931 British physicist Alan Herries Wilson establishes the band theory of conduction. AFTER 1958–59 American engineers Jack Kilby and Robert Noyce develop the integrated circuit. 1971 Intel Corporation releases the first microprocessor. E lectronics encompasses the science, technology, and engineering of components and circuits for the generation, transformation, and control of electrical signals. Electronic circuits contain active components, such as diodes and transistors, that switch, amplify, filter, or otherwise change electrical signals. Passive components, such as cells, lamps, and motors, typically only convert between electrical and other forms of energy, such as heat and light. The term “electronics” was first applied to the study of the electron, the subatomic particle that carries electricity in solids. The discovery of the electron in 1897 by the

ELECTRICITY AND MAGNETISM 153 See also: Electric charge 124–127 ■ Electric current and resistance 130–133 ■ Quantum applications 226–231 ■ Subatomic particles 242–243 Electronics is the use of To produce these signals, Semiconductors contain electricity to produce signals the flow of electric a variable number of that carry information and current must be controlled charge carriers (particles control devices. with great precision. that carry electricity). In most semiconductors, the most important Doping (adding impurities to) electronic component, the transistor, is made silicon semiconductors affects the nature of their charge carriers, enabling current of doped silicon regions. It can amplify electrical signals or turn them on or off. to be controlled precisely. British physicist J.J. Thomson surface and move through the into DC (direct current). This generated scientific study of how tube. When a voltage was allowed for the detection of AC the particle’s electric charge could applied across the electrodes, it radio waves, so the valve found be harnessed. Within 50 years, this made the anode relatively positive, wide application as a demodulator research had led to the invention so the electrons were attracted (detector) of signals in early of the transistor, paving the way to it and a current flowed. When AM (amplitude modulation) for the concentration of electrical the voltage was reversed, the radio receivers. signals into ever-more compact electrons emitted by the cathode devices, processed at ever-greater were repelled from the anode In 1906, American inventor speeds. This led to the spectacular and no current flowed. The diode Lee de Forest added a third, grid- advances in electronic engineering conducted only when the anode shaped electrode to Fleming’s and technology of the digital was positive relative to the diode to create a triode. A small, revolution that began in the late cathode, and so acted as a one- varying voltage applied across the 20th century and continue today. way valve, which could be used new electrode and the cathode to convert AC (alternating current) changed the electron flow between Valves and currents the cathode and anode, creating The first electronic components The Colossus Mark II computer a large voltage variation—in other were developed from vacuum of 1944 used a vast number of valves words, the small input voltage was tubes—glass tubes with the air to make the complex mathematical amplified to create a large output removed. In 1904, British physicist calculations needed to decode the voltage. The triode became a vital John Ambrose Fleming developed Nazi Lorenz cipher systems. component in the development of the vacuum tube into the thermionic radio broadcasting and telephony. diode, which consisted of two electrodes—a cathode and an Solid-state physics anode—inside the tube. The Although in the following decades metal cathode was heated by an valves enabled further advances in electrical circuit to the point of technology, such as the television thermionic emission, whereby and early computers, they were its negatively charged electrons bulky, fragile, power-hungry, and gained enough energy to leave the limited in the frequency of their operation. The British Colossus ❯❯

154 ELECTRONICS It is at a surface where many conduct current. In conductors, boron, had a small deficiency. It of our most interesting and the valence and conduction bands became clear that tiny amounts useful phenomena occur. overlap, so electrons involved in of impurities in a semiconductor bonding can also contribute to crystal can dramatically change its Walter Brattain conduction. In insulators, there electrical properties. The controlled is a large band gap, or energy introduction of specific impurities to computers of the 1940s, for difference, between the valence obtain desired properties became example, each had up to 2,500 and conduction bands that keeps known as “doping.” valves, occupied a whole room, most electrons bonding but not and weighed several tons. conducting. Semiconductors have The regions of a crystal can be a small band gap. When given a doped in different ways. In a pure All of these limitations were little extra energy (by applying silicon crystal, for example, each resolved with the transition to heat, light, or voltage), their valence atom has four bonding (valence) electronics based not on vacuum electrons can migrate to the electrons that it shares with its tubes but on the electron properties conduction band, changing the neighbors. Regions of the crystal of semiconducting solids, such as properties of the material from can be doped by introducing some the elements boron, germanium, insulator to conductor. atoms of phosphorus (which has and silicon. This in turn was born five valence electrons) or boron out of an increasing interest from Control through doping (with three valence electrons). the 1940s onward in solid-state In 1940, a chance discovery added The phosphorus-doped region has physics—the study of those another dimension to the electrical extra “free” electrons and is called properties of solids that depend potential of semiconductors. When an n-type semiconductor (n for on microscopic structure at the testing a crystal of silicon, Russell negative). The boron-doped region, atomic and subatomic scales, Ohl, an American electrochemist, called a p-type semiconductor including quantum behavior. found that it produced different (p for positive) has fewer electrons, electrical effects if probed in creating charge-carrying “holes.” A semiconductor is a solid different places. When examined, When the two types are joined, with electrical conductivity varying the crystal appeared to have it is called a p–n junction. If a between that of a conductor and an regions that contained distinct voltage is applied to the p-type insulator—so neither high nor low. impurities. One, phosphorus, had a side, it attracts electrons from In effect, it has the potential to small excess of electrons; another, the n-type side and current flows. control the flow of electric current. A crystal with one p–n junction Electrons in all solids occupy distinct energy levels grouped Most semiconductors in transistors are made of silicon (Si), which into bands, called valence and has been doped with impurities to control the flow of current through conduction bands. The valence it. Adding phosphorus atoms to silicon makes an n-type semiconductor, band contains the highest energy with negatively charged electrons that are free to move. Adding boron level that electrons occupy when atoms makes a p-type semiconductor, with positively charged “holes” bonding with adjacent atoms. The that can move through the silicon. conduction band has even higher energy levels, and its electrons are Transistor Positively charged hole not bound to any particular atoms, but have enough energy to move Silicon electron throughout the solid, and so Extra electron from Si Si Si Si Boron atom phosphorus atom has one less electron, which acts as a “hole,” attracting a silicon electron Phosphorus Si P Si B Boron atom atom N-type P-type (negative) silicon (positive) silicon

ELECTRICITY AND MAGNETISM 155 John Bardeen Born in 1908, John Bardeen was 1957 he coauthored the BCS just 15 when he began electrical (Bardeen–Cooper–Schrieffer) engineering studies at the local theory of superconductivity. University of Wisconsin. After He shared two Nobel Prizes graduating, he joined the Gulf in Physics—in 1956 (for the Oil laboratories in 1930 as a transistor) and again in 1972 geophysicist, developing magnetic (for BCS theory). He died in 1991. and gravitational surveys. In 1936, he earned a PhD in mathematical Key works physics from Princeton University, and later researched solid-state 1948 “The transistor, a semi- physics at Harvard. During World conductor triode” War II, he worked for the US Navy 1957 “Microscopic theory of on torpedoes and mines. superconductivity” 1957 “Theory of After a productive period superconductivity” at Bell Labs, where Bardeen coinvented the transistor, in acts as a diode—it conducts a which was achieved by wrapping the device soon drove spectacular current across the junction in gold foil around the corner of a growth in the electronics market only one direction. piece of plastic and slitting the foil and began to replace vacuum along the edge to create two tightly tubes in computers. Transistor breakthroughs spaced contacts. When they were After World War II, the search for pressed onto the germanium, they Processing power an effective replacement for the formed an amplifier that could The first transistors were made vacuum tube, or valve, continued. boost the signal that was fed to from germanium, but this was In the US, the Bell Telephone the base. This first working version superseded as a base material Company assembled a team of was simply known as a “point by the much more abundant and American physicists, including contact” but soon became known versatile silicon. Its crystals form on William Shockley, John Bardeen, as a “transistor.” their surface a thin insulating layer and Walter Brattain at its of oxide. Using a technique called laboratories in New Jersey to The point-contact transistor photolithography, this layer can be develop a semiconductor version was too delicate for reliable high- engineered with microscopic of the triode amplifier. volume production, so in 1948 precision to create highly complex Shockley began work on a new patterns of doped regions and other Bardeen was the main theorist, transistor. Based on the p–n features on the crystal. and Brattain the experimenter, in semiconductor junction, Shockley’s the group. After failed attempts at “bipolar” junction transistor was The advent of silicon as a base applying an external electric field to built on the assumption that the and advances in transistor design a semiconductor crystal to control positively charged holes created by drove rapid miniaturization. This its conductivity, a theoretical doping penetrated the body of the led first to integrated circuits breakthrough by Bardeen shifted semiconductor rather than just (entire circuits on a single crystal) the focus onto the surface of the floating across its surface. The in the late 1960s, and in 1971 to semiconductor as the key site transistor consisted of a sandwich the Intel 4004 microprocessor—an of changes in conductivity. In 1947, of p-type material between two entire CPU (central processing unit) the group began to experiment n-type layers (npn), or n-type on one 3  4 mm chip, with more with electrical contacts on top of a sandwiched between p-type layers than 2,000 transistors. Since then, crystal of germanium. A third (pnp), with the semiconductors the technology has developed with electrode (the “base”) was attached separated by p–n junctions. By incredible rapidity, to the point underneath. For the device to work 1951, Bell was mass-producing the where a CPU or GPU (graphics as desired, the two top contacts transistor. Although at first mainly processing unit) chip can contain needed to be very close together, used in hearing aids and radios, up to 20 billion transistors. ■

156 EALNEIMCTARLICITY BIOELECTRICITY IN CONTEXT B ioelectricity enables the Richard Caton recorded the entire nervous system of an electrical fields produced by the KEY FIGURES animal to function. It allows brains of rabbits and monkeys. Joseph Erlanger (1874–1965), the brain to interpret heat, cold, Herbert Spencer Gasser danger, pain, and hunger, and to The breakthrough in (1888–1963) regulate muscle movement, understanding how the electrical including heartbeat and breathing. impulses were produced came BEFORE in 1932, when Joseph Erlanger 1791 Italian physicist Luigi One of the first scientists to and Herbert Gasser found that Galvani publishes his findings study bioelectricity was Luigi different fibers in the same nerve on “animal electricity” in a Galvani, who coined the term cord possess different functions, frog’s leg. “animal electricity” in 1791. He conduct impulses at different rates, observed muscular contraction in and are stimulated at different 1843 German physician Emil a dissected frog’s leg when a cut intensities. From the 1930s onward, du Bois-Reymond shows that nerve and a muscle were connected Alan Hodgkin and Andrew Huxley electrical conduction travels with two pieces of metal. In 1843, used the giant axon (part of a along nerves in waves. Emil du Bois-Reymond showed that nerve cell) of a veined squid to nerve signals in fish are electrical, study how ions (charged atoms AFTER and in 1875, British physiologist or molecules) move in and out 1944 American physiologists of nerve cells. They found that Joseph Erlanger and Herbert when nerves pass on messages, Gasser receive the Nobel Prize sodium, potassium, and chloride for Physiology or Medicine for ions create fast-moving pulses of their work on nerve fibers. electrical potential. ■ 1952 British scientists Alan Sharks and some other fish have Hodgkin and Andrew Huxley jelly-filled pores called ampullae of show that nerve cells Lorenzini, which detect changes in communicate with other electric fields in the water. These cells by means of flows of sensors can detect a change of just ions; this becomes known as 0.01 microvolt. the Hodgkin-Huxley model. See also: Magnetism 122–123 ■ Electric charge 124–127 ■ Electric potential 128–129 ■ Electric current and resistance 130–133 ■ Making magnets 134–135

ELECTRICITY AND MAGNETISM 157 UDSAINCTSEIOCEXTNOPATVELIECFLRTIYCYED STORING DATA IN CONTEXT C omputer hard-disk drives We had just participated in (HDD) store data encoded the birth of the phenomenon KEY FIGURES into bits, written on the Albert Fert (1938–), disk surface as a series of changes which we called giant Peter Grünberg (1939–2018) in direction of magnetization. The magnetoresistance. data is read by detecting these Albert Fert BEFORE changes as a series of 1s and 0s. 1856 Scottish physicist The pressure to store more data in nonmagnetic material between William Thomson (Lord Kelvin) less space has driven a constant magnetic layers only a few atoms discovers magnetoresistance. evolution in HDD technology, but a thick, and by applying small major problem soon emerged: it was magnetic fields, the current flowing 1928 The quantum- difficult for conventional sensors to through became spin-polarized. mechanical idea of electron read ever-greater amounts of data The electron spin was either spin is postulated by Dutch– on ever-smaller disk space. oriented “up” or “down,” and if the American physicists George magnetic field changed, the spin- Uhlenbeck and Samuel In 1988, two teams of computer polarized current was switched on Goudsmit. scientists—one led by Albert Fert, or off, like a valve. This spin-valve the other by Peter Grünberg— could detect minute magnetic 1957 The first computer independently discovered giant impulses when reading HDD data, hard-disk drive (HDD) is the magnetoresistance (GMR). GMR allowing vastly increased amounts size of two refrigerators, and depends on electron spin, a of data to be stored. ■ can store 3.75 megabytes (MB) quantum-mechanical property. of data. Electrons possess either up-spin AFTER or down-spin—if an electron’s spin 1997 British physicist is “up,” it will move easily through Stuart Parkin applies an up-oriented magnetized giant magnetoresistance material, but it will encounter to create extremely sensitive greater resistance through a down- spin-valve technology for oriented magnet. The study of data-reading devices. electron spin became known as “spintronics.” By sandwiching a See also: Magnetism 122–123 ■ Nanoelectronics 158 ■ Quantum numbers 216–217 ■ Quantum field theory 224–225 ■ Quantum applications 226–231

158 OOANNF AETNHPCEINYHCELAODPEDIA NANOELECTRONICS IN CONTEXT T he components that are In 1975, he revised the timescale integral to virtually every to every two years, a maxim that KEY FIGURE electronic device, from became known as “Moore’s law.” Gordon Moore (1929–) smartphones to car ignition systems, Although the rate of miniaturization are only a few nanometers (nm) has slowed since 2012, the smallest BEFORE in scale (1 nm is one-billionth of 1 modern transistors are 7 nm across, Late 1940s The earliest meter). A handful of tiny integrated enabling 20 billion transistor-based transistors are made, circuits (ICs) can perform functions circuits to be integrated into measuring 1⁄3 inch in length. that were previously carried out by a single computer microchip. thousands of transistors, switching Photolithography (transferring a 1958 American electrical or amplifying electronic signals. pattern from a photograph onto the engineer Jack Kilby ICs are collections of components, semiconducting material) is used demonstrates the first such as transistors and diodes, that to fabricate these nanocircuits. ■ working integrated circuit. are printed onto a silicon wafer, a semiconducting material. 1959 Richard Feynman challenges other scientists Reducing the size, weight, and to research nanotechnology. power consumption of electronic devices has been a driving trend AFTER since the 1950s. In 1965, American 1988 Albert Fert and Peter engineer Gordon Moore forecast the Grünberg independently demand for ever-smaller electronic discover the GM (giant components and predicted that the magnetoresistance) effect, number of transistors per silicon enabling computer hard drives chip would double every 18 months. to store ever-greater amounts of data. Gordon Moore, who was CEO of the technology company Intel between 1999 American electronics 1975 and 1987, is best known for his engineer Chad Mirkin invents observations about the demand for dip-pen nanolithography, ever-smaller electronic components. which “writes” nanocircuitry See also: The motor effect 136–137 ■ Electronics 152–155 ■ Storing data 157 on silicon wafers. ■ Quantum applications 226–231 ■ Quantum electrodynamics 260

ELECTRICITY AND MAGNETISM 159 EOAIRTSHISNEOGRULTNEHOPROTLHE, MAGNETIC MONOPOLES IN CONTEXT I n classical magnetism, magnets Experimenters will continue have two poles that cannot to stalk the monopole with KEY FIGURES be separated. If a magnet is Gerard ’t Hooft (1946–), broken in half, the broken end simply unfailing determination Alexander Polyakov (1945–) becomes a new pole. However, and ingenuity. in particle physics, magnetic John Preskill BEFORE monopoles are hypothetical 1865 James Clerk Maxwell particles with a single pole, American theoretical physicist unifies electricity and either north or south. Theoretically, magnetism in one theory. opposite magnetic monopoles nuclear forces merge into a single attract, matching monopoles repel, force. In 1974, theoretical physicists 1894 Pierre Curie suggests and their trajectories bend in Gerard ’t Hooft and Alexander that magnetic monopoles an electric field. Polyakov independently argued could exist. that GUT predicts the existence There is no observational or of magnetic monopoles. 1931 Paul Dirac proposes that experimental proof that magnetic magnetic monopoles could monopoles exist, but in 1931, British In 1982, a detector at Stanford explain the quantization of physicist Paul Dirac suggested that University recorded a particle electric charge. they could explain the quantization consistent with a monopole, but of electric charge, whereby all such particles have never been 1974 American physicists electron charges are multiples found since, despite scientists’ Sheldon Glashow and Howard of 1.6  10–19 coulombs. efforts to find them using highly Georgi publish the first Grand sensitive magnetometers. ■ Unified Theory (GUT) in Gravity, electromagnetism, particle physics. the weak nuclear force, and the strong nuclear force are the four AFTER recognized fundamental forces. 1982 Blas Cabrera, a In particle physics, several variants Spanish physicist at Stanford of Grand Unified Theory (GUT) University, California, records propose that in an exceptionally an event consistent with a high-energy environment, monopole passing through electromagnetic, weak, and strong a superconducting device. See also: Magnetism 122–123 ■ Electric charge 124–127 ■ Quantum field theory 224–225 ■ Quantum applications 226–231 ■ String theory 308–311

LSIOGUHNTD AN the properties of waves

D

162 INTRODUCTION Pythagoras discovers Euclid writes Optics, Dutch astronomer Willebrord Robert Hooke publishes the link between the asserting that light Snellius devises a law for Micrographia, the first length of lyre strings travels in straight the relationship between lines and describing an angle of a ray of study of minute and the pitch of the law of reflection. objects viewed through sound they make. light entering a transparent material and its angle the microscopes of refraction. he designed. 6TH CENTURY BCE 3RD CENTURY CE 1621 1665 1658 4TH CENTURY BCE 50 CE Aristotle correctly suggests that Hero of Alexandria shows that Pierre de Fermat shows that all sound is a wave conveyed the law of reflection can be laws of reflection and refraction derived using geometry alone through motion of the air but by adopting the rule that light can be described using the erroneously proposes that high always travels between two principle that light always frequencies travel faster than points by the shortest path. travels between two points on the path that takes the least time. low frequencies. H earing and sight are the explored the effects of vibrating reflection and refraction: light senses we use most to lyre strings of different tensions always chooses to take the interpret the world; little and lengths. shortest path it can between wonder that light—a prerequisite any two points. for human sight—and sound Reflection and refraction have fascinated humans since Mirrors and reflections have also The phenomena of reflection the dawn of civilization. long been things of wonder. Again and refraction of light allowed in ancient Greece, scholars proved— pioneers such as Italian physicist Music has been a feature of using geometry alone—that light and astronomer Galileo Galilei and everyday life since Neolithic always returns from a mirrored Robert Hooke in England to create times, as cave paintings and surface at the same angle at which devices that offered new views archaeology testify. The ancient it approaches it, a principle known of the universe, while microscopes Greeks, whose love of learning today as the law of reflection. unveiled nature. Telescopes infused every element of their revealed hitherto unseen moons culture, sought to work out the In the 10th century, Persian orbiting other planets and principles behind the production mathematician Ibn Sahl noted the prompted a reassessment of of harmonic sounds. Pythagoras, relationship between a ray of light’s Earth’s place in the cosmos, while inspired by hearing hammers angle of incidence as it entered a microscopes offered a view into an strike different notes on an anvil, transparent material and its angle alien world of tiny creatures and recognized that a relationship of refraction within that material. the cellular structure of life itself. existed between sound and the In the 17th century, French size of the tool or instrument mathematician Pierre de Fermat The very nature of light was that produced it. In a number proposed correctly that a single questioned by influential scientists of studies, he and his students principle pertained in both many times over the centuries. Some felt that light was composed

SOUND AND LIGHT 163 Isaac Newton German-born British Christian Doppler Northern Irish hypothesizes that astronomer William describes the change in the astrophysicist Herschel discovers Jocelyn Bell Burnell light is made infrared radiation, color of stars in relation discovers pulsars up of particles the first light outside to their motion, an effect (pulsating stars) called corpuscles. the electromagnetic later demonstrated using using radio spectrum. sound waves. astronomy. 1675 1800 1842 1967 1669 1690 1803 1865 Danish scientist Rasmus Dutch physicist Christiaan Thomas Young’s James Clerk Maxwell Bartholin describes Huygens publishes Treatise experiment splitting demonstrates that light polarization a beam of sunlight after observing on Light, outlining his light is an oscillation the birefringence of wave theory and how light demonstrates the of magnetic and calcite crystals. bends as it passes between wave properties electric fields. different media. of light. of small particles which move light were acting as a wave, and their relative motion as one moved through the air from a source to an the waves each side of the thin away and the other toward Earth. observer, possibly after reflection card interfered with one another. The theory was proven for sound and from an object. Others saw light as deemed correct for light; the color a wave, citing behavior such Wave theory took off, but only of light depends on its wavelength, as diffraction (spreading of light as when it became clear that light which is shorter as a star approaches it passed through narrow gaps). traveled in transverse waves, unlike and longer when it is moving away. English physicists Isaac Newton sound, which travels in longitudinal and Robert Hooke opposed each waves. Soon, physicists examining In the 19th century, physicists other; Newton favored particles the properties of light also noticed also discovered new light invisible and Hooke opted for waves. that light waves could be forced to to the human eye—first infrared oscillate in particular orientations and then ultraviolet. In 1865, British polymath and physician known as polarization. By 1821, Scottish physicist James Clerk Thomas Young provided an answer. French physicist Augustin-Jean Maxwell interpreted light as an In an experiment outlined to the Fresnel had produced a complete electromagnetic wave, prompting Royal Society in 1803, he split a theory of light in terms of waves. questions about how far the beam of sunlight, dividing it by electromagnetic spectrum might means of a thin card, so that it The Doppler effect extend. Soon physicists discovered diffracted and produced a pattern While considering the light from light with more extreme frequencies, on a screen. The pattern was not pairs of stars orbiting each other, such as X-rays, gamma rays, radio that of two bright patches from two Austrian physicist Christian Doppler waves, and microwaves—and their rays of light, but a series of bright noticed that most pairs showed one multiple uses. Light that is invisible and dark repeating lines. The star red and the other blue. In 1842, to humans is now an essential part results could only be explained if he proposed that this was due to of modern life. ■

164 IN CONTEXT OGTTHHFEOEETRMHHEEEUTIMSSRMTYRIININNGGS KEY FIGURE Pythagoras (c. 570–495 bce) MUSIC BEFORE c. 40,000 bce The earliest known purpose-built musical instrument is a flute carved from a vulture’s wing bone, found at the Hohle Fels cave near Ulm, Germany, in 2008. AFTER c. 350 bce Aristotle suggests that sound is conveyed by the movement of air. 1636 French theologian and mathematician Marin Mersenne discovers the law linking the fundamental frequency of a stretched string to its length, mass, and tension. 1638 In Italy, Galileo Galilei claims that pitch depends on the frequency, rather than the speed, of sound waves. H umans have been making music since prehistoric times, but it was not until the golden age of ancient Greece (500–300 bce) that some of the physical principles behind the production of harmonic sounds, in particular frequency and pitch, were established. The earliest scientific attempt to understand these fundamental aspects of music is generally attributed to the Greek philosopher Pythagoras. The sound of music Pitch is determined by the frequency of sound waves (the number of waves that pass a fixed point each second). A combination

SOUND AND LIGHT 165 See also: The scientific method 20–23 ■ The language of physics 24–31 ■ Pressure 36 ■ Harmonic motion 52–53 ■ Electromagnetic waves 192–195 ■ Piezoelectricity and ultrasound 200–201 ■ The heavens 270–271 The pitch of a musical The timbre (sound Limiting the note depends on the quality) of a musical movement of a string frequency of its note depends on the at specific fractions of its sound waves. distinct shape of its length produces notes that sound waves. create a pleasing musical scale. Music is governed by patterns in sound waves and mathematical ratios. of physics and biology enables or even sung by another singer. to test the hammers and anvils, its perception by the human ear. Musicians can also alter the timbre and in doing so discovered a Today, we know that sound waves of an instrument by using different relationship between the size of are longitudinal—they are created playing techniques. A violinist, for the hammer used and the pitch by air being displaced back and example, can change the way the of the sound it produced. forth in directions parallel to that strings vibrate by using the bow in which the wave is propagating in different ways. Like many other stories (moving through the medium that about Pythagoras, this episode carries it). We perceive sound Pythagorean findings is certainly invented (there is no when these oscillations cause According to legend, Pythagoras relationship between pitch and our eardrums to vibrate. formulated his ideas about pitch the size of the hammer striking while listening to the melodic an anvil) but it is true that the While the frequency or pitch sound of hammers striking anvils philosopher and his followers found of a sound dictates the note that as he passed a busy blacksmith’s fundamental links between the we hear, music also has another workshop. Hearing a note that physics of musical instruments quality known as “timbre.” This sounded different from the others, and the notes they produced. is the subtle variation in the rise he is said to have rushed inside They realized that there was a and fall of the waves produced by relationship between the size and ❯❯ a particular musical instrument— features of the wave’s “shape” Even if two notes have the same pitch, their sound depends that are distinct from its simple on the shape of their waves. A tuning fork produces a pure frequency and wavelength (the sound with just one pitch. A violin has a jagged waveform distance between two successive with pitches called overtones on top of its fundamental pitch. peaks and troughs in a wave). No nondigital musical instrument Simple waveform Overtone can produce sound waves that vary consistently and entirely smoothly, and timbre is the reason why a note sung by a human voice can sound very different when it is played on a stringed, wind, or brass instrument,

166 MUSIC Harmony … depends on “first harmonic”. Standing waves fundamental tone and the octave musical proportion; it is of shorter wavelengths, known as through a series of seven steps nothing but a mysterious “higher harmonics,” can be created without striking discordant notes. by “stopping” a string (holding or The 3:2 ratio defined the fifth of musical relation. limiting its movement at another these steps, and became known Florian Cajori point on its length). The “second as the “perfect fifth.” harmonic” is produced by stopping Swiss–American mathematician the string precisely halfway along A set of seven musical notes its length. This results in a wave (equivalent to the white keys, sound of musical instruments. whose entire wavelength matches designated A–G, of an octave on In particular, they found similar the string length—in other a modern piano) proved somewhat relationships among the sounds words, the wavelength is half and limiting, however, and smaller produced by strings of different the frequency double that of the tuning divisions called semitones lengths, organ pipes of varying fundamental tone. To human were later introduced. These led to heights, and wind instruments of hearing, this creates a note with a versatile system of twelve notes different diameters and lengths. many of the same characteristics (both the white and black keys on as the fundamental tone but with a modern piano). While the seven The Pythagoreans’ discoveries a higher pitch—in musical terms it white keys alone can only make had more to do with systematic is an octave higher. A stop one- a pleasing progression (known as experiments involving vibrating third of the way down the string the “diatonic scale”) when played strings than hammers and anvils. creates the third harmonic, with a in order up or down from one “C” The observation that shorter strings wavelength of two-thirds the length to the next, the additional black vibrate more quickly and produce of the string and a frequency three keys (“sharps” and “flats”) allow higher notes was nothing new— times that of the fundamental tone. such a progression to be followed it had been the basis of stringed from any point. instruments for at least 2,000 years. The perfect fifth However, vibrating identical strings The difference between the second “Pythagorean tuning” based in different ways produced more and third harmonics was important. on the perfect fifth was used to interesting results. Equivalent to a 3:2 ratio between the find the desirable pitch of notes on frequencies of vibrating waves, it Western musical instruments for Pythagoras and his students separated pitches (musical notes) many centuries, until changes in tested strings from lyres (an early that blended together pleasingly, musical taste in the Renaissance musical instrument) with varying but were also musically more era led to more complex tunings. lengths and tensions. Plucking a distinct from each other than Musical cultures outside Europe, string in the middle of its length, harmonic notes separated by a such as those of India and China, for example, creates a “standing whole octave. wave” in which the middle oscillates back and forth while Experimentation soon allowed the ends remain fixed in place. In the Pythagoreans to construct effect, the string produces a wave an entire musical system around with a wavelength that is twice this relationship. By building and its own length and a frequency correctly “tuning” other strings determined by the wavelength and to vibrate at frequencies set by the tension in the string. This is simple numerical relationships, known as the fundamental tone or they constructed a musical bridge, or progression, between the The lyre of ancient Greece originally had four strings, but had acquired as many as 12 by the 5th century bce. Players plucked the strings with a plectrum.

SOUND AND LIGHT 167 A medieval woodcut depicts the musical investigations of Pythagoras and his follower Philolaus, including the various sounds produced by different sized wind instruments. followed their own traditions, and Aristotle and passed into the Pythagoras although they, too, recognized the canon of Western pre-Renaissance pleasing effects when notes of musical theory. Little is known about the early certain pitches were played in life of Pythagoras, but later sequence or together. Another long-standing musical Greek historians agreed that misunderstanding passed down he was born on the Aegean Music of the spheres? from the Pythagoreans was their island of Samos, a major For Pythagorean philosophers, claim that the pitch of a string trading center, in around the realization that music was had a proportional relationship 570 bce. Some legends tell of shaped by mathematics revealed a to both its length and to the youthful travels in the Near profound truth about the universe tension at which it was strung. East, and studying under as a whole. It inspired them to When Italian lute-player and Egyptian or Persian priests look for mathematical patterns musical theorist Vincenzo Galilei and philosophers, as well as elsewhere, including in the (father of Galileo) investigated Greek scholars. They also say heavens. Studies of the cyclic these supposed laws in the that on returning to Samos, patterns in which the planets and mid-16th century, he found that he rose rapidly in public life. stars moved across the sky led to while the claimed relationship a theory of cosmic harmony that between length and pitch was At around the age of 40, later became known as the “music correct, the law of tension Pythagoras moved to Croton, of the spheres.” was more complex—the pitch a Greek city in southern Italy, varied in proportion to the square where he founded a school of Several followers of Pythagoras root of the tension applied. philosophy that attracted also attempted to explain the many followers. Surviving nature of musical notes by Galilei’s discovery led to wider writings from Pythagoras’s considering the physics involved. questions about the supposed pupils suggest that his The Greek philosopher Archytas superiority of ancient Greek teachings included not (c. 428–347 bce) suggested that knowledge, while his experimental only mathematics and music oscillating strings created sounds method—carrying out practical but also ethics, politics, that moved at different speeds, tests and mathematical analysis metaphysics (philosophical which humans heard as different rather than taking the claims of inquiry into the nature of pitches. Although incorrect, this authority for granted—became reality itself), and mysticism. theory was adopted and repeated a major influence on his son. ■ in the teachings of the hugely Pythagoras acquired great influential philosophers Plato They saw in numbers the political influence over the attributes and ratios of leaders of Croton and may the musical scales. have died during a civil Aristotle uprising that was triggered by his rejection of calls for a on the Pythagoreans democratic constitution. His death is placed at around 495 bce.

168 LLTIEHGAEHSPTTAFTTOHIMLOLEFOWS REFLECTION AND REFRACTION IN CONTEXT R eflection and refraction surface, known as the “normal.” The are the two fundamental angle between the incoming ray KEY FIGURE behaviors of light. and the normal is the same as Pierre de Fermat (1607–1665) Reflection is the tendency for that between the normal and the light to bounce off a surface in a reflected ray. In the 1st century ce, BEFORE direction related to the angle at the mathematician Hero of c. 160 ce Ptolemy puts forward which it approaches that surface. Alexandria showed how this a theory that the eye emits Early studies led the Greek path always involves the light rays which return information mathematician Euclid to note ray traveling over the shortest to the observer. that light is reflected off a mirror distance (and spending the at an “angle of reflection” equal to least time doing so). c. 990 Persian mathematician its “angle of incidence”—the angle Ibn Sahl develops a law of at which it approaches in relation to Refraction is the way rays of light refraction after studying the a line perpendicular to the mirror’s change direction when passing bending of light. from one transparent material to 1010 Arabic scholar Ibn Light approaching the boundary to another material can either be al-Haytham’s Book of Optics reflected off at the same angle to the “normal” line, perpendicular to the proposes that vision is the surface, or refracted at an angle that relates to the angle of approach and result of rays entering the eye. the relative speed of light in the two materials. Whether light is reflected or refracted, it always follows the shortest and simplest path. AFTER 1807 Thomas Young coins Normal Refracted ray as the term “refractive index” to denser secondary describe the ratio of the speed Incident ray Angle of Angle of material slows of light in a vacuum to its incidence incidence it down speed in a refracting material. and reflection are equal Secondary 1821 Frenchman Augustin material Fresnel outlines a complete theory of light and describes Incident refraction and reflection in ray terms of waves. Normal Angle of refraction Reflected Ray exits secondary ray material and returns to original speed Secondary material Reflection Refraction

SOUND AND LIGHT 169 See also: Energy and motion 56–57 ■ Focusing light 170–175 ■ Lumpy and wavelike light 176–179 ■ Diffraction and interference 180–183 ■ Polarization 184–187 ■ The speed of light 275 ■ Dark matter 302–305 another. It was 10th-century Persian A landscape is reflected in a lake on entering a denser material, mathematician Ibn Sahl who first on a still day. The angle of reflection and away from it when entering found a law linking a ray’s angle equals the angle of incidence—the one that is less dense. of incidence at the boundary of angle at which sunlight reflected two materials with its angle of from the landscape hits the water. Fermat’s discovery was refraction in the second material, important in its own right, but as well as the properties of the two smallest imaginable wavelength. also widely regarded as the first materials. This was rediscovered in This is often seen as justification example of what is known as a Europe in the early 17th century, for the concept of a light “ray”— family of “variational principles” most notably by Dutch astronomer the widely used idea of a beam of in physics, describing the tendency Willebrord Snellius, and is known light that travels through space of processes to always follow the as Snell’s law. along the path of least time. When most efficient pathway. ■ light is moving through a single Shortest path and time unchanging medium, this means In 1658, French mathematician that it will travel in straight lines Pierre de Fermat realized that both (except when the space through reflection and refraction could be which it travels is distorted). described using the same basic However, when light reflects from principle—an extension of that a boundary or passes into a put forward by Hero of Alexandria. transparent medium in which Fermat’s principle states that the its speed is slower or faster, this path light will take between any “principle of least time” dictates two points is that which can be the path it will take. crossed in the least time. By assuming that the speed Fermat created his principle of light was finite, and that light by considering Dutch physicist moved more slowly in denser Christiaan Huygens’ earlier transparent materials, Fermat was concept describing the motion able to derive Snell’s law from his of light in the form of waves and principle, describing correctly how how it applied to cases with the light will bend toward the normal Pierre de Fermat Born in 1607, the son of a merchant the energy required. This from southwestern France, Pierre was important for understanding de Fermat trained and worked not only light and large-scale as a lawyer, but is mostly motion, but also the behavior remembered for mathematics. of atoms on the quantum level. He found a means of calculating Fermat died in 1665; three the slopes and turning points of centuries later, his famous curves a generation before Isaac Fermat’s last theorem about Newton and Gottfried Leibniz the behavior of numbers at used calculus to do so. powers higher than 2 was finally proved in 1995. Fermat’s “principle of least time” is regarded as a key step Key work toward the more universal “principle of least action”—an 1636 “Method of Finding the observation that many physical Maxima, Minima and Linear phenomena behave in ways that Tangents of Curves” minimize (or sometimes maximize)

WORLDA NEW VISIBLE FOCUSING LIGHT



172 FOCUSING LIGHT J ust as rays of light can a larger part of the eye’s field of be reflected in different vision. These magnifying lenses, IN CONTEXT directions by plane mirrors however, had limitations. Studying (with a flat surface), or refracted objects that were very close to KEY FIGURE from one angle to another as they the lens, using larger lenses to Antonie van Leeuwenhoek cross a flat boundary between achieve a greater field of view (1632–1723) different transparent materials, (area of the object that could be so curved surfaces can be used magnified) involved bending light BEFORE to bend light rays onto converging rays that would be diverging 5th century bce In ancient paths—bringing them together at strongly between opposite sides Greece, convex disks of crystal a point called a focus. The bending of the lens. This required a more or glass are used to ignite fires and focusing of light rays using powerful lens (sharply curved and by focusing sunlight on a lenses or mirrors is the key to thicker at the center), which due single point. many optical instruments, including to the limitations of early glass the groundbreaking single-lens manufacture was more likely c. 1000 ce The first lenses, microscope designed by Antonie to produce distortions. High glass “reading stones” with van Leeuwenhoek in the 1670s. magnifications therefore seemed a flat base and convex upper impossible to achieve. These surface, are used in Europe. Magnifying principles problems held up the development The first form of optical instrument of optical instruments for centuries. AFTER was the lens, used in Europe from 1893 German scientist August the 11th century. The refraction The first telescopes Köhler devises an illumination process as light enters and leaves In the 17th century, scientists system for the microscope. a convex lens (with an outward- realized that by using a curving surface) bends the combination of multiple lenses 1931 In Germany, Ernst Ruska spreading light rays from a source instead of a single lens in an and Max Knoll build the first onto more closely parallel paths, instrument they could improve electron microscope, which creating an image that occupies magnification significantly, uses the quantum properties of electrons to view objects. While the original “Galilean” telescope design uses Only part a concave eyepiece lens to bend light outward without of the light 1979 The Multiple Mirror passing through a focus, the more advanced Keplerian from the Telescope at Mount Hopkins, design uses a convex lens beyond the focus to produce image enters Arizona, is built. an image covering a larger field of view. the pupil Light ray Objective lens Where the telescope Galilean refracting telescope Concave eyepiece ends, the microscope Light ray Objective lens Light from the entire begins. Which of image enters the pupil the two has the grander view? Focal point Victor Hugo Les Misérables Convex eyepiece Keplerian refracting telescope

SOUND AND LIGHT 173 See also: Reflection and refraction 168–169 ■ Lumpy and wavelike light 176–179 ■ Diffraction and interference 180–183 ■ Polarization 184–187 ■ Seeing beyond light 202–203 Lens-shaped glass or Lens shapes can also However, attempting crystal can bend sunlight bend light from nearby to magnify nearby objects by large amounts to create a focus. objects to create a creates distortions due magnified image. to lens thickness. A single beadlike lens that is Combining an objective lens completely spherical can with another eyepiece can produce a magnified image produce an even more highly of a small area. magnified image of a tiny area. creating optical tools that could In 1609, Italian physicist Galileo In 1611, German astronomer magnify not only nearby objects Galilei built his own telescope Johannes Kepler came up with but also those very far away. based on this model. His rigorous a better design. In the Keplerian approach allowed him to improve telescope, the rays are allowed The first compound optical on the original, producing devices to meet, and the eyepiece lens is instrument (one using multiple that could magnify by a factor of convex rather than concave. Here, lenses) was the telescope, usually 30. These telescopes allowed him the lens is placed beyond the focus said to have been invented by to make important astronomical point, at a distance where rays Dutch lens-maker Hans Lippershey discoveries, but the images were have begun to diverge again. around 1608. Lippershey’s device, still blurry, and the field of view tiny. The eyepiece therefore acts more consisting of two lenses mounted like a normal magnifying glass, at either end of a tube, produced [T]he effect of creating an image with a larger images of distant objects magnified my instrument is such field of view and potentially offering by a factor of three. Just as a that it makes an object fifty far higher magnification. Keplerian magnifying glass bends diverging miles off appear as large telescopes form images that are rays from nearby objects onto upside-down and back-to-front, but more parallel paths, so the front, as if it were only this was not a significant problem or “objective.” lens of a telescope five miles away. for astronomical observations, or gathers near-parallel rays from more Galileo Galilei even for practiced terrestrial users. distant objects and bends them onto a converging path. However before Tackling aberration the rays can meet, a concave The key to increasing a telescope’s (inward-curving) “eyepiece” lens magnifying power lay in the bends them back onto diverging strength and separation of its paths—creating an image in the lenses, but stronger lenses observer’s eye that appears larger brought with them problems of and (because the objective lens their own. Astronomers identified gathered much more light than two major problems—colorful ❯❯ a human pupil) also brighter.

174 FOCUSING LIGHT Ten times more powerful than the Hubble Space Telescope, the Giant Magellan Telescope in Chile, due to be completed in 2025, will capture light from the outer reaches of the universe. fringes around objects known as same idea allowed telescope intercepted the converging rays “chromatic aberration” (caused by makers to eliminate spherical and sent them toward an eyepiece different colors of light refracting aberration, too. on the side of the telescope tube at different angles) and blurred (crossing over at a focus point images due to “spherical aberration” Mirrored telescopes along the way). (difficulties in making a lens of From the 1660s, astronomers ideal curvature). began using reflector telescopes. Using reflection rather than These use a concave, curved refraction sidestepped the issue It seemed that the only practical primary mirror to gather parallel of chromatic aberration, but early route to high magnification with light rays and bounce them onto reflectors were far from perfect. minimal aberration was to make converging paths. Various designs The “speculum metal” (a highly the objective lens larger and were proposed in the early 17th polished copper and tin alloy), used thinner, and place it much further century, but the first practical for mirrors of the time, produced away from the eyepiece. In the late example was built by Isaac Newton faint images tinted with brassy 17th century, this led to the building in 1668. This used a spherically colors. Silver-coated glass, a of extraordinary “aerial telescopes” curved primary lens, with a small better reflector, was pioneered focusing light over distances of secondary mirror suspended at a by French physicist Léon Foucault more than 330 ft (100 meters). diagonal angle in front of it, which in the 1850s. A more practical solution was Compound microscopes discovered by British lawyer Just as scientists combined lenses Chester Moore Hall in around to magnify distant objects, they 1730. He realized that nesting did the same to improve the power an objective lens made of weakly of the magnifying glass to look at refractive glass next to a concave minuscule objects that were second lens with much stronger close. Credit for inventing the refraction would create a “doublet” “compound microscope” is that focused all colors at a single disputed, but Dutch inventor distance, avoiding blurry fringes. Cornelis Drebbel demonstrated This “achromatic” lens became such an instrument in London widespread after Hall’s technique in 1621. In order to minimize was discovered and marketed thickness and reduce aberration, by optician John Dollond in the 1750s. Later developments of the Secondary Main light- mirror gathering lens Eyepiece Primary light- Eyepiece Reflector gathering mirror lens telescopes use curved mirrors to Reflector Refractor gather light and telescope telescope produce an image. Refractor telescopes use only lenses.

1665 book Micrographia, caused a SOUND AND LIGHT 175 sensation, but his work was soon surpassed by that of Dutch scientist Antonie van Antonie van Leeuwenhoek. Leeuwenhoek My method for seeing the very Simple ingenuity Born in the Dutch town of Delft smallest animalcules and Van Leeuwenhoek’s single-lens in 1632, van Leeuwenhoek microscope was far more powerful became an apprentice in a minute eels, I do not impart to than compound microscopes of linen shop at the age of six. others; nor how to see very the time. The lens itself was a tiny On marrying in 1654, he set glass bead—a polished sphere able up his own draper’s business. many animalcules at one time. to bend light rays from a small area Wishing to study the quality Antonie van very sharply to create a highly of fibers up close, but unhappy Leeuwenhoek magnified image. A metal frame, with the power of available holding the tiny lens in place, was magnifying glasses, he the objective lens of a compound held close to the eye, in order for studied optics and began to microscope has a very small the sky to act as a light source make his own microscopes. diameter (so light rays passing behind it. A pin on the frame held through different points on the the sample in place, and three Van Leeuwenhoek soon lens are only slightly divergent). screws could move the pin in three started to use his microscopes The eyepiece lens, which is often dimensions, allowing adjustments for scientific studies, and a larger than the objective, does to the focus and the area on which physician friend, Regnier de most of the work of producing the light focused by the lens fell. Graaf, brought his work to the the magnified image. attention of London’s Royal Van Leeuwenhoek kept his Society. Van Leeuwenhoek’s The most successful early lens-making technique a secret, studies of the microscopic compound microscope was but it achieved a magnification world, including his discovery designed by Robert Hooke in the of at least 270 times, revealing of single-celled organisms, 1660s. He mounted a third convex features far smaller than Hooke amazed its members. Elected lens between the objective lens had seen. With this powerful tool, to the Royal Society in 1680, and the eyepiece, bending the light he was able to discover the first he was visited by leading more sharply and increasing the bacteria, human spermatozoa, and scientists of the day until magnification, at a cost of greater the internal structure of the “cells” his death in 1723. aberration. He also addressed already identified by Hooke, for another important issue—the which he is known as the founding Key works tiny field of view on which the father of microbiology. ■ microscope focused meant that 375 letters in Philosophical images tended to be dim (simply Van Leeuwenhoek made his Transactions of the Royal because there are fewer light rays microscopes himself. They comprised Society reflecting from a smaller area). a tiny spherical lens held between two 27 letters in Memoirs of To correct this, he added an plates with a hole in each side and a the Paris Academy of Sciences artificial source of illumination— pin to hold the specimen. a candle whose light was focused onto the object by a spherical glass bulb filled with water. Hooke’s large-format drawings of plant and insect structures, created by using the microscope and published in the influential

176 IN CONTEXT LAIGWHATVEIS KEY FIGURE Thomas Young (1773–1829) LUMPY AND WAVELIKE LIGHT BEFORE 5th century bce The Greek philosopher Empedocles claims that “light corpuscles” emerge from the human eye to illuminate the world around it. c. 60 bce Lucretius, a Roman philosopher, proposes that light is a form of particle emitted by luminous objects. c.1020 In his Book of Optics, Arab polymath Ibn al-Haytham theorizes that objects are lit by the reflection of light from the sun. AFTER 1969 At Bell Labs, in the US, Willard Boyle and George E. Smith invent the charge- coupled device (CCD), which generates digital images by collecting photons. T heories about the nature of light had occupied the minds of philosophers and scientists since ancient times, but it was the development of optical instruments such as the telescope and microscope in the 1600s that led to important breakthroughs, such as confirmation that Earth was not the center of the solar system and the discovery of a microscopic world. In around 1630, French scientist and philosopher René Descartes, seeking to explain the phenomenon of light refraction (how light bends when it passes between media), proposed that light was a traveling disturbance—a wave moving at

SOUND AND LIGHT 177 See also: Force fields and Maxwell’s equations 142–147 ■ Diffraction and interference 180–183 ■ Polarization 184–187 ■ Electromagnetic waves 192–195 ■ Energy quanta 208–211 ■ Particles and waves 212–215 White light splits into the rainbow experiments—in which he One may conceive light to components of the visible spectrum split white light into its color spread successively, by when it passes through a prism. The components using a prism and spherical waves. precise color depends on the length then recreated a white beam Christiaan Huygens of the wave—red has the longest. using lenses—revealed the truth. bends when it passes between infinite speed through a medium Newton also investigated different media (such as water that filled empty space. (He called reflection. He showed that beams and air) in terms of wave behavior. this medium the plenum.) Around of light always reflect in straight Huygens rejected the corpuscular 1665, English scientist Robert lines and cast sharp-edged model on the grounds that two Hooke was the first to make a link shadows. In his view, wavelike light beams could collide without between the diffraction of light (its would show more signs of bending scattering in unexpected ability to spread out after passing and spreading, so he concluded directions. He suggested that light through narrow openings) and the that light must be corpuscular— was a disturbance moving at very similar behavior of water waves. made up of tiny, lumpy particles. high (though finite) speed through This led him to suggest that not Newton published his findings in what he called the “luminiferous only was light a wave, but it was a 1675 and developed them further ether”—a light-carrying medium. ❯❯ transverse wave, like water—one in in his book Opticks in 1704. His which the direction of disturbance premise dominated theories about is at right angles to the direction light for more than a century. of motion, or “propagation.” Light bends The color spectrum Newton’s work had some failings, Hooke’s ideas were largely particularly when it came to overlooked thanks to the influence refraction. In 1690, Dutch scientist within the scientific community and inventor Christiaan Huygens of his great rival, Isaac Newton. published Treatise on Light, in In around 1670, Newton began a which he explained how light series of experiments on light and optics in which he showed that Water waves produce a Light spreads out color is an intrinsic property of spreading semicircular after passing through light. Until then, most researchers had assumed that light acquired pattern after passing a narrow slit. colors through its interaction with through a narrow opening. different matter, but Newton’s Overlapping waves in Overlapping light waves water interfere with each interfere with each other to other to produce complex produce lighter and patterns. darker regions. Light is a wave.

178 LUMPY AND WAVELIKE LIGHT The experiments I am about tank—a shallow bath of water with the way they illuminated a screen to relate … may be repeated a paddle at one end to generate to how a single beam illuminated with great ease, whenever the periodic waves. Placing obstacles, it. A pattern of light and dark areas sun shines, and without any such as a barrier with one or more appeared on the screen when the openings, in the bath allowed dividing card was in place, but other apparatus than is at him to study wave behavior. He disappeared when the card was hand to every one. showed how after passing through removed. Young had elegantly Thomas Young a narrow slit in a barrier, parallel shown that light was behaving in straight waves would produce a the same way as water waves— Huygens developed a useful spreading semicircular pattern— as the two separated beams principle in which each point on an effect similar to the diffraction diffracted and overlapped, areas the advancing “wave front” of a of light through narrow openings. of greater and lesser intensity light beam is treated as a source of interacted to produce an interference smaller “wavelets” spreading in all A barrier with two slits created pattern that the corpuscular theory directions. The overall course of the a pair of spreading wave patterns could not explain. (The experiment beam can be predicted by finding that overlapped. Waves from the was later refined with two narrow the direction in which the wavelets two slits could freely pass through slits in an opaque glass surface, align and reinforce each other. each other, and where they crossed, for which it became known as the wave height was determined the “double-slit experiment”.) Young’s experiments by the phases of the overlapping Young argued that his For most of the 18th century, the waves—an effect known as demonstration showed beyond corpuscular model dominated interference. When two wave peaks doubt that light was a wave. thinking on light. That changed or two troughs overlapped, they What was more, the patterns in the early 19th century with the increased the height or depth of the varied according to the color of work of British polymath Thomas wave; when a peak and a trough the diffracted light, which implied Young. As a medical student in met, they cancelled each other out. that colors were determined the 1790s, Young investigated the by wavelengths. properties of sound waves and Young’s next step, outlined in became interested in general wave a lecture to the Royal Society in While Young’s experiment phenomena. Similarities between 1803, was to demonstrate that light revived debate about the nature sound and light convinced him behaved in a similar way to waves. of light, the wave theory of light that light might also form waves, To do this, he split a narrow beam was not yet fully accepted. Despite and in 1799 he wrote a letter of sunlight into two beams using Hooke’s early ideas, most scientists setting out his argument to a piece of thin card and compared the Royal Society in London. The classic “double-slit” experiment, a modification Waves cancel Faced with fierce skepticism of Thomas Young’s original, produces a pair of each other out from the followers of Newton’s diffracting waves whose interference pattern (peaks and troughs model, Young set about devising generates light and dark stripes on a screen. intersect), producing experiments that would prove the the dark stripes wave behavior of light beyond Dividing card doubt. With a shrewd grasp of the with two slits power of analogy, he built a ripple Interference pattern of light and dark stripes on the screen Light Waves come together source (peaks intersect with peaks Incoming and troughs intersect with light rays troughs), producing the white stripes

SOUND AND LIGHT 179 Thomas Young The eldest of 10 children, Young of Egyptian hieroglyphs, was born into a Quaker family in explained the working of the the English county of Somerset eye, investigated the properties in 1773. A born polymath, he of elastic materials, and mastered several modern and developed a method for tuning ancient languages as a child, musical instruments. His death before going on to study medicine. in 1829 was widely mourned in After making a mark with his the scientific community. private scientific research, in 1801 he was appointed professor at the Key works recently founded Royal Institution, but later resigned to avoid conflict 1804 “Experiments and with his medical practice. Calculations Relative to Physical Optics” Young continued to study and 1807 A Course of Lectures on experiment and made valuable Natural Philosophy and the contributions in many fields. He Mechanical Arts made progress in the translation (including Young himself) at light supposedly traveled. Most now called photons. The intensity first assumed that if light was a experts believed that Maxwell’s of a light source depends on the wave, it must be longitudinal— equations described the speed number of photons it produces, like sound, where the disturbance at which light would enter the but the energy of an individual of the medium moves back and ether from a source. The failure photon depends on its wavelength forth parallel to the direction of of increasingly sophisticated or frequency—hence high-energy propagation. This made some experiments designed to detect blue photons can provide electrons light phenomena, such as this light-carrying medium raised with enough energy to flow, while polarization, impossible to fundamental questions and created lower-energy red ones, even in explain through waves. a crisis that was only resolved by large numbers, cannot. Since the Einstein’s theory of special relativity. early 20th century, the fact that A solution emerged in France light can behave like a wave or in 1816, when André-Marie Ampère Waves and particles like a particle has been confirmed suggested to Augustin-Jean Einstein was largely responsible for in numerous experiments. ■ Fresnel that the waves might be a final shift in our understanding transverse, which would explain of light. In 1905, he put forward an [W]e have good reason to the behavior of polarized light. explanation for the photoelectric conclude that light itself […] Fresnel went on to produce a effect—a phenomenon in which detailed wave theory of light that currents flow from the surface is an electromagnetic also explained diffraction effects. of certain metals when they are disturbance in the form of exposed to certain kinds of light. Electromagnetic properties Scientists had been puzzled by waves propagating […] Acceptance of light’s wavelike the fact that a feeble amount of according to the laws nature coincided with developing blue or ultraviolet light would of electromagnetism. studies of electricity and magnetism. cause a current to flow from some James Clerk Maxwell By the 1860s, James Clerk Maxwell metals while they remained was able to describe light in a inactive under even the most series of elegant equations as an intense red light. Einstein electromagnetic disturbance moving suggested (building on the concept at 186,282 miles (299,792 km) of light quanta previously used by per second. Max Planck) that despite being fundamentally wavelike, light However, awkward questions travels in small particle-like bursts, still surrounded Huygens’ so-called “luminiferous ether” through which

180 IN CONTEXT NLTTIHOEGVEHBESTERHNIKSADNDIOONWWTON KEY FIGURE Augustin-Jean Fresnel DIFFRACTION AND INTERFERENCE (1788–1827) BEFORE 1665 Robert Hooke compares the movement of light to the spreading of waves in water. 1666 Isaac Newton demonstrates that sunlight is composed of different colors. AFTER 1821 Augustin-Jean Fresnel publishes his work on polarization, suggesting for the first time that light is a transverse (waterlike) wave rather than a longitudinal (soundlike) wave. 1860 In Germany, Gustav Kirchhoff and Robert Bunsen use diffraction gratings to link bright “emission lines” of light at specific wavelengths with different chemical elements. D ifferent types of waves share similar behaviors, such as reflection (bouncing off a surface at the same angle as they approach the surface), refraction (changing direction at the boundaries between mediums), and diffraction—the way a wave spreads around obstacles or spreads out when it passes through an aperture. An example of diffraction is the way water waves spread into the shadow region beyond a barrier. The discovery that light also exhibits diffraction was key to proving its wavelike nature. The diffraction of light was first systematically observed in the 1660s by Francesco Maria

SOUND AND LIGHT 181 See also: Reflection and refraction 168–169 ■ Lumpy and wavelike light 176–179 ■ Polarization 184–187 ■ The Doppler effect and redshift 188–191 ■ Electromagnetic waves 192–195 ■ Particles and waves 212–215 Incoming light allowed it to bend. It was Grimaldi Like light waves, wind-generated rays travel straight who coined the term “diffraction” waves on a lake spread out in circular to describe this. His findings, ripples when they pass through a Outgoing light published posthumously in 1665, narrow gap, illustrating the property diffracts were a thorn in the side of of diffraction common to all waves. scientists trying to prove the When linear light waves pass corpuscular nature of light. Neither the wave theory of light through a narrow aperture in a nor Newton’s corpuscular theory barrier, they diffract (spread out) Competing theories explained the phenomenon of into semicircular wave fronts. In the 1670s, Isaac Newton colored fringing—something that treated diffraction (which he Grimaldi had noted around both Grimaldi, a Jesuit priest and called “inflexion”) as a special the edges of the illuminated circle physicist in Italy. Grimaldi type of refraction that occurred and the shadow of the rod in his built a dark room with a pinhole when light rays (which he believed experiments. Newton’s struggle aperture through which a pencil- were corpuscular) passed close with this question led him to width beam of sunlight could to obstacles. At around the same propose some intriguing ideas. enter and hit an angled screen. time, Newton’s rival Robert He maintained that light was Where the beam struck the screen, Hooke cast doubt on his theory strictly corpuscular, but that each it created an oval circle of light, by conducting successful fast-moving corpuscle could act as which Grimaldi measured. He demonstrations of Grimaldi’s a source of periodic waves whose then held a thin rod in the path experiments, and Dutch scientist vibrations determined its color. of the light beam and measured and inventor Christiaan Huygens the size of the shadow it cast put forward his own wave theory Young’s experiments within the illuminated area. of light. Huygens argued that many Newton’s improved theory of light, Grimaldi then compared his optical phenomena could only be published in his book Opticks in results to calculations based on explained if light was treated as 1704, did not entirely resolve the assumption that light travels an advancing “wave front,” along the questions around diffraction, in straight lines. He found that which any point was a source but the corpuscular model was not only was the shadow larger of secondary waves, whose widely accepted until 1803, when than such calculations suggested interference and reinforcement Thomas Young’s demonstration it should be, but so was the determined the direction in of interference between diffracted illuminated oval. which the wave front moved. light waves led to a resurgence of ❯❯ The conclusion Grimaldi drew Nature is not embarrassed from these comparisons was that by difficulties of analysis. light was not composed of simple She avoids complication particles (corpuscles) that traveled in straight lines, but had wavelike only in means. properties, similar to water, that Augustin-Jean Fresnel

182 DIFFRACTION AND INTERFERENCE Augustin-Jean interest in Huygens’ ideas. Young is a distance where red light is still Fresnel proposed two modifications to diffracted, but blue light is not, then the Huygens’ model that could it is red fringing that would appear. The second of four sons of explain diffraction. One was the a local architect, Fresnel was idea that the points on a wave front Crucial breakthrough born at Broglie in Normandy, at the very edges of the open In 1818, the French Academy France, in 1788. In 1806, aperture produced wavelets that of Sciences offered a prize for a he enrolled to study civil spread into the “shadow” region full explanation of the “problem engineering at the National beyond the barrier. The other of inflection” identified by Young. School of Bridges and Roads, was that the observed pattern of Civil engineer Augustin-Jean and went on to become a diffraction came about through a Fresnel had been working on government engineer. wave passing near the edge of the precisely this question for several aperture interfering with a wave years, conducting a series of Briefly suspended from bouncing off the sides of the barrier. intricate homemade experiments work for his political views and sharing his findings with during the last days of the Young also proposed that there academician François Arago. Some Napoleonic Wars, Fresnel must be a distance where light of his work unwittingly replicated pursued his interest in optics. was sufficiently far from the edge that of Young, but there were Encouraged by physicist to be unaffected by diffraction. If new insights as well, and Arago François Arago, he wrote this distance varied between the encouraged Fresnel to submit an essays and a prize memoir different colors of light, Young explanatory memoir for the prize on the subject, including a maintained, it could explain the competition. In this memoir, mathematical treatment of colored fringes discovered by Fresnel gave complex mathematical diffraction. Fresnel later Grimaldi. For instance, if there explained polarization using a model of light as a transverse Light falls into areas Light is bending or wave and developed a lens for that should be completely spreading out as it focusing intense directional passes obstructions. beams of light (mainly used in in shadow. lighthouses). He died in 1827, at the age of 39. The wave front in Huygens’ model would produce secondary waves that might spread out into shaded areas. Secondary wave fronts perfectly reinforce or cancel out only in a few places—in most, their interference is more complex. Key works Light diffraction Interference patterns are only is a wavelike completely dark where 1818 Memoir on the Diffraction behavior. waves cancel out each of Light other—elsewhere, light is 1819 “On the Action of Rays always present. of Polarized Light upon each other” (with François Arago)

SOUND AND LIGHT 183 The colors on a soap bubble are judges, Siméon-Denis Poisson, bird feather artificially by winding caused by the interruption of light pointed out, Fresnel’s equations fine hairs between two tightly waves as they reflect off the thin predicted that the shadow cast by threaded screws to create a mesh film of the bubble and interfere a circular obstacle illuminated by a with a density of around 100 hairs with one another. point source of light, such as a per inch. This device was perfected pinhole, should have a bright spot in 1813 by German physicist and equations to describe the position at its very center—an idea Poisson instrument-maker Joseph von and strength of dark and light believed to be absurd. The matter Fraunhofer, who named it the interference “fringes” and showed was resolved when Arago promptly “diffraction grating.” Crucially, that they matched the results of constructed an experiment that diffraction gratings offer a far more real experiments. produced just such a “Poisson efficient means of splitting light spot.” Despite this success, the than glass prisms because they Fresnel also demonstrated wave theory was not broadly absorb very little of the light that the geometry of the fringes accepted until 1821, when Fresnel striking them. They are therefore depended on the wavelength of published his wave-based approach ideal for producing spectra from the light that had produced them. to explain polarization—how faint sources. Gratings and related For the first time, it was possible transverse light waves sometimes instruments have gone on to be to measure interference fringes align in a particular direction. extremely useful in many branches produced by a monochromatic of science. (single-colored) light source and Diffraction and dispersion thereby calculate the wavelength While Young and Fresnel were Fraunhofer also built the first of that specific color. learning to understand the effects “ruling engine”—a machine that of diffraction and interference by painstakingly scratched narrowly In the memoir, Fresnel summed directing light beams through spaced transparent lines into a up the difference between Huygens’ two narrow slits, some instrument- glass plate covered with an wave theory and his own with makers were taking a different otherwise opaque surface layer. remarkable simplicity. Huygens’ approach. As early as 1673, This device allowed him to spread error, he said, was to assume that Scottish astronomer James Gregory sunlight into a far wider spectrum light would only spread where had noted that sunlight passing than could previously be achieved, the secondary waves exactly through the fine gaps in a bird’s revealing the presence of what reinforced each other. In reality, feather spreads out into a rainbow- became known as “Fraunhofer however, complete darkness is only like spectrum similar to the lines,” which would later play a key possible where the waves exactly behavior of white light passing role in understanding the chemistry cancel each other out. through a prism. of stars and atoms. ■ Despite its elegance, Fresnel’s In 1785, American inventor I think I have found the prize memoir faced a skeptical David Rittenhouse succeeded in explanation and the law of reception from a judging panel who replicating the effect of Gregory’s colored fringes which one mostly supported the corpuscular notices in the shadows of theory of Newton. As one of the bodies illuminated by a luminous point. Augustin-Jean Fresnel

184 IN CONTEXT STTAHHINDEEDENRSSOAOOYRUFTTHH KEY FIGURE Étienne-Louis Malus POLARIZATION (1775–1812) BEFORE 13th century References to the use of “sunstones” in Icelandic sagas suggest that Viking sailors may have used the properties of Iceland spar crystals to detect polarization for daytime navigation. AFTER 1922 Georges Friedel investigates the properties of three kinds of “liquid crystal” material and notes their ability to alter the polarization plane of light. 1929 American inventor and scientist Edwin H. Land invents “Polaroid,” a plastic whose polymer strands act as a filter for polarized light, only transmitting light in one plane. P olarization is the alignment of waves in a specific plane or direction. The term is usually applied to light, but any transverse wave (oscillating at right angles to the wave direction) can be polarized. A number of natural phenomena produce light that is polarized—a portion of the sunlight reflected off a flat and shiny surface such as a lake, for example, is polarized to match the angle of the lake’s surface—and light can also be polarized artificially. The investigation of different effects caused by polarization helped determine that light is a wavelike phenomenon, and also provided important evidence

SOUND AND LIGHT 185 See also: Force fields and Maxwell’s equations 142–147 ■ Focusing light 170–175 ■ Lumpy and wavelike light 176–179 ■ Diffraction and interference 180–183 ■ Electromagnetic waves 192–195 Light energy consists of vibrating electric and magnetic fields. I believe that this In normal light In polarized phenomenon can serve lovers rays, these fields vibrate light, each field of nature and other interested oscillates in a persons for instruction or at in planes at random constant plane. angles. least for pleasure. Rasmus Bartholin on birefringence The planes of the fields can remain fixed in orientation or rotate. that it is electromagnetic in nature. an early champion of the wave and Newton could not envisage The first account of identical light theory, argued that the speed of how these could be sensitive beams differing for seemingly light rays traveling through the to direction. inexplicable reasons was given by crystal varied according to the Sides and poles Danish scientist Rasmus Bartholin direction in which they moved, In the early 19th century, French in 1669. He discovered that and in 1690 used his “wave front” soldier and physicist Étienne-Louis viewing an object through principle to model the image- Malus carried out his own version the crystals of a transparent doubling effect. of Huygens’ experiments. Malus mineral known as Iceland spar was interested in applying ❯❯ (a form of calcite) produced a Huygens also carried out some double image of that object. new experiments, placing a second Sunglasses with polarized lenses crystal of Iceland spar in front of reduce the harsh glare of light reflecting This phenomenon, known the first and rotating it. When he off a snowy landscape by allowing only as birefringence, arises because did so, he found that the doubling light waves that have been polarized in the crystal has a refractive index effect disappeared at certain one direction to pass through. (speed at which the light is angles. He did not understand transmitted) that changes why, but he recognized that the according to the polarization of two images produced by the first the light passing through it. Light crystal were different from each reflected from most objects has other in some way. no particular polarization, so on average, half the rays are split Isaac Newton maintained that along each path, and a double birefringence strengthened his image is created. case for light as a corpuscle—a particle with discrete “sides” that Early explanations could be affected by its orientation. Bartholin published a detailed In Newton’s view, the effect discussion of the birefringence helped disprove the wave theory, effect in 1670, but he could not as Huygens’ model of light involved explain it in terms of any particular longitudinal waves (disturbances model of light. Christiaan Huygens, parallel to the wave direction) rather than transverse waves,

186 POLARIZATION mathematical rigor to the study Have not the rays of light in a bowl of water as he rotated of light. He developed a useful several sides, endued with the crystal—the reflections would mathematical model for what was disappear and reappear depending actually happening to light rays several properties? on the rotation of the crystal. He in three dimensions when they Isaac Newton soon developed a law (Malus’s law) encountered materials with describing how the intensity of a different reflective and refractive Malus discovered an important polarized image viewed through properties. (Previous models had new aspect of the phenomenon a crystal “filter” relates to the simplified matters by considering while using a piece of Iceland spar orientation of the crystal. only what happened to light rays to observe a reflection of the setting moving in a single flat plane— sun in a window of the Palais du Further experiments revealed two dimensions.) Luxembourg in Paris. He noticed a similar effect with reflections that the intensity of the sun’s two from transparent materials. Malus In 1808, the French Institute images varied as he rotated the noticed that interaction between of Science and Arts offered a crystal, and one or other image the surface of the materials and prize for a fullexplanation of disappeared completely with every unpolarized light (with a random birefringence and encouraged 90-degree turn of the crystal. This, mix of orientations) reflected light Malus to participate. Like many in Malus realized, meant that the in one specific plane of polarization the French scientific establishment, sunlight had already been polarized rather than others, while light Malus followed the corpuscular by reflection off the glass. He polarized in the other plane passed theory of light, and it was hoped confirmed the effect by shining into or through the new medium that a corpuscular explanation for candlelight through a birefringent (such as water or glass) along a birefringence might help debunk crystal and looking at how the refracted path. Malus realized that the wave theory recently put resulting pair of light rays reflected the factor determining whether to forward in Britain by Thomas reflect or refract must be the internal Young. Malus developed a theory structure of the new medium, linked in which light corpuscles had to its refractive index. distinct sides and “poles” (axes of rotation). Birefringent materials, he New insights claimed, refracted corpuscles along Malus’s identification of a link different paths depending on the between a material’s structure direction of their poles. He coined and its effect on polarized light was the term “polarization” to describe important: one common application this effect. today is to study the internal Étienne-Louis Malus Born into a privileged Parisian the “caustic” reflections created family in 1775, Malus showed by curved reflecting surfaces, early mathematical promise. He impressed his peers, and in 1810 served as a regular soldier before his explanation of birefringence attending the École Polytechnique led to him being elected to the engineering school in Paris from Académie des Sciences. A year 1794, after which he rose through later, he was awarded the Royal the ranks of the army engineering Society’s Rumford Medal for the corps and took part in Napoleon’s same work, despite Britain being expedition to Egypt (1798–1801). at war with France. Malus died On returning to France, he worked in 1812, aged 36. on military engineering projects. Key works From 1806, a posting to Paris enabled Malus to mix with 1807 Treatise on Optics leading scientists. His talent for 1811 “Memoir on some mathematically describing the new optical phenomena” behaviors of light, including

SOUND AND LIGHT 187 changes in materials under stress Vertical filter accepts Horizontal filter blocks from the way they affect polarized only vertical rays vertical rays light. He attempted to identify a relationship between a material’s Nothing refractive index and the angle at passes which light reflecting from a through surface would be perfectly “plane the filter polarized” (aligned in a single plane). He found the correct angle Unpolarized light Vertical for water but was thwarted by the at right angles polarized light poor quality of other materials he investigated; a general law was Unpolarized light waves oscillate at right angles to their direction found a few years later in 1815 by of propagation (motion) in random directions. Plane-polarized light Scottish physicist David Brewster. waves oscillate in a single direction, while the plane of circular- polarized waves rotates steadily as they propagate. The range of phenomena involving polarization grew. In 1811, light polarized along the other axis. wave. André-Marie Ampère French physicist François Arago Using dichroic materials would suggested to Fresnel that a solution found that passing polarized light become one of the ways in which might be found by treating light through quartz crystals could rotate polarized light could be easily as a transverse wave. If this were its axis of polarization (an effect now produced. Others included using the case, it followed that the axis known as “optical activity”), and his certain types of glass and specially of polarization would be the plane contemporary Jean-Baptiste Biot shaped calcite prisms. in which the wave was oscillating reported that light from rainbows (vibrating). Thomas Young reached was highly polarized. Polarized waves the same conclusion when Arago In 1816, Arago and his protégé told him about the experiment, but Biot went on to identify optical Augustin-Jean Fresnel made it was Fresnel who took inspiration activity in liquids, formulating the an unexpected and important to create a comprehensive wave concept of circular polarization (in discovery that undermined claims theory of light, which would which the polarization axis rotates that polarization supported the eventually displace all older models. as the light ray moves forward). He corpuscular nature of light. They also discovered “dichroic” minerals, created a version of Thomas Polarization would also play a which are natural materials that Young’s “double-slit” experiment key role in the next major advance allow light to pass through if it is using two beams of light whose in the understanding of light. In polarized along one axis, but block polarization could be altered, and 1845, British scientist Michael found that patterns of dark and light Faraday, seeking a way to prove We find that light acquires caused by interference between the the suspected link between light properties which are two beams were strongest when and electromagnetism, decided they had the same polarization. But to see what would happen if he relative only to the sides the patterns faded as the difference shone a beam of polarized light of the ray … I shall give the in polarization increased, and they through a magnetic field. He disappeared completely when the discovered that he could rotate name of poles to these polarization planes were at right the plane of polarization by sides of the ray. angles to each other. altering the strength of the field. This phenomenon, known as the Étienne-Louis Malus This discovery was inexplicable Faraday effect, would ultimately through any corpuscular theory of inspire James Clerk Maxwell to light, and could not be explained by develop his model of light as an imagining light as a longitudinal electromagnetic wave. ■

188 IN CONTEXT WTTARHNAEUDVMETPHTEERTAEINRS KEY FIGURE Christian Doppler THE DOPPLER EFFECT AND REDSHIFT (1803–1853) BEFORE 1727 British astronomer James Bradley explains the aberration of starlight—a change in the angle at which light from distant stars approaches Earth, caused by Earth’s motion around the sun. AFTER 1940s “Doppler radar” is developed for aviation and weather forecasting after early forms of radar fail to account for rainfall and the Doppler shifts of moving targets. 1999 Based on observations of exploding stars, astronomers discover that some distant galaxies are further away than their Doppler shifts suggest, implying that the expansion of the universe is accelerating. T oday, the Doppler effect is a familiar part of our everyday lives. We notice it as the shift in the pitch of sound waves when a vehicle with an emergency siren speeds toward us, passes us, and continues into the distance. The effect is named after the scientist who first proposed it as a theoretical prediction. When applied to light, it has turned out to be a powerful tool for learning about the universe. Explaining star colors The idea that waves (specifically, waves of light) might change their frequency depending on the relative motion of the source and the

SOUND AND LIGHT 189 See also: Lumpy and wavelike light 176–179 ■ Discovering other galaxies 290–293 ■ The static or expanding universe 294–295 ■ Dark energy 306–307 [The Doppler effect] will in the a higher frequency than they Christian Doppler not too distant future offer otherwise would, so the light’s measured wavelength is shorter. Born in the Austrian city of astronomers a welcome means In contrast, when the source and Salzburg in 1803 to a wealthy to determine the movements observer move apart, peaks in the family, Doppler studied and distances of such stars. wave reach the observer at lower mathematics at the Vienna frequency than they otherwise Polytechnic Institute, then Christian Doppler would. The color of light is took physics and astronomy dependent on its wavelength, so as a postgraduate. Frustrated observer was first proposed by light from an approaching object at the bureaucracy involved in Austrian physicist Christian appears bluer (due its shorter securing a permanent post, Doppler in 1842. Doppler was wavelengths) than it otherwise he came close to abandoning investigating the aberration of would, while light from a retreating academia and emigrating to starlight—a slight shift in the object appears redder. the US, but eventually found apparent direction of light from employment as a math teacher distant stars, caused by Earth’s Doppler hoped this effect (which in Prague. motion around the sun—when he was soon named after him) would realized that motion through space help explain the different colors Doppler became a professor would cause a shift in the frequency, of stars in the night sky when at the Prague Polytechnic in and therefore color, of light. viewed from Earth. He elaborated 1838, where he produced a his theory to take into account the wealth of mathematics and The principle behind the theory motion of Earth through space, but physics papers, including a is as follows. When the source and did not realize the magnitude by celebrated study on the colors observer approach one another, which the speed of stellar motions of stars and the phenomenon “peaks” in the light waves from is dwarfed by the speed of light, now known as the Doppler the source pass the observer at making the effect undetectable effect. By 1849, his reputation with the instruments of the time. was such that he was given an important post at the However, Doppler’s discovery University of Vienna. His was confirmed in 1845, thanks to an health—delicate throughout ingenious experiment with sound. much of his life—deteriorated, Making use of the recently opened and he died of a chest Amsterdam–Utrecht railroad, Dutch infection in 1853. scientist C.H.D. Buys Ballot asked a group of musicians to play a single ❯❯ Key work Sound waves spread out at the same speed in all 1842 Über das farbige Licht directions when measured relative to the source. But der Doppelsterne if a siren approaches an observer, the wave frequency (Concerning the Colored Light increases and the wavelength is shortened. of Double Stars) Sound waves from a siren are compressed Once the ambulance has passed, the as the ambulance nears, reducing their siren’s sound waves stretch, increasing wavelength and raising their pitch their wavelength and lowering their pitch

190 THE DOPPLER EFFECT AND REDSHIFT An emergency vehicle’s The light of stars looks appear in the spectrum of a static siren sounds lower-pitched redder as stars move star. In 1868, William Huggins successfully measured the Doppler once the vehicle has away from Earth. shift of the spectral lines of Sirius, passed an observer. the brightest star in the sky. The frequency of sound or light waves Glimpsing deeper changes depending on the relative motion of In the late 19th century, astronomy’s reliance on painstaking observations the source and observer. was transformed by improvements in photography, enabling scientists This is called the Doppler effect. to measure the spectra and Doppler shifts of much fainter stars. Long- continuous note while traveling on narrow, dark lines—such as those exposure photography gathered the train. As the carriage sped past already observed in the sun’s vastly more light than the human him, Buys Ballot experienced the spectrum, and presumed to exist eye, and yielded images that could now-familiar pitch-shift of sound— in the spectra of other stars—that be stored, compared, and measured the high pitch of the note as the could act as defined reference long after the original observation. train approached him grew lower points in a spectrum of light. as the train traveled away. German astronomer Hermann Putting this idea into practice, Carl Vogel pioneered this marriage Measuring the shift however, required significant of photography and “spectroscopy” The effects of the Doppler shift advances in technology. At such in the 1870s and 1880s, leading to on light were calculated by French vast distances, even the brightest important discoveries. In particular, physicist Armand Fizeau in 1848. stars are far more faint than the he identified many apparently Unaware of Doppler’s own work, he sun, and the spectra created when single stars with spectral lines that provided a mathematical basis for their light is split by diffraction—in periodically “doubled,” separating the concept of red and blue light order to measure individual spectral and then reuniting in a regular shifts. Doppler had suggested that lines—are fainter still. British cycle. He showed that this line- his theory would bring about a new astronomers William and Margaret splitting arose because the stars understanding of the motions, Huggins measured the spectrum of are binary pairs—visually distances, and relationships of light from the sun in 1864, and their inseparable stars whose close stars, but Fizeau suggested a colleague William Allen Miller was orbits around each other mean practical way in which shifts in able to use the measurements to that as one star retreats from Earth starlight might be detected. He identify elements in distant stars. correctly recognized that changes [The] color and intensity in a star’s overall color would be By this time, German scientists of […] light, and the pitch both minute and hard to quantify. Gustav Kirchhoff and Robert and strength of a sound will Instead, he proposed that Bunsen had shown that the sun’s be altered by a motion of the astronomers could identify what Fraunhofer lines were caused by we know as Doppler shifts through specific elements absorbing light, source of the light the positions of “Fraunhofer lines,” which enabled astronomers to or of the sound … calculate the wavelengths where William Huggins these absorption lines would

SOUND AND LIGHT 191 So-called “cosmological” redshift is not caused by galaxies cooks. The rate of expansion is the moving apart in a conventional sense, but by the stretching of space same tiny amount for each light itself as the universe has expanded over billions of years. This carries year of space, but it accumulates galaxies apart from each other and stretches the wavelengths of light as across the vastness of the universe it moves between them. until galaxies are dragged apart at close to the speed of light, and Seen through a Galaxy moves wavelengths of light stretch into telescope, receding further away the far red—and beyond visible galaxy appears red light, into the infrared—end of the to observer spectrum. The relationship between distance and redshift (denoted z) is so direct that astronomers use z, not light years, to indicate distance for the furthest objects in the universe. Telescope took this as evidence that spiral Practical applications nebulae were vast, independent Doppler’s discovery has had many Wavelength of galaxies, located far beyond the technological applications. Doppler light stretches gravitational grasp of the Milky radar (used in traffic-control “radar Way. However, it was not until 1925 guns” and air-security applications) its light appears more red, while that American astronomer Edwin reveals the distance to—and relative the other approaches Earth and Hubble calculated the distance to speed of—an object that reflects a its light appears more blue. spiral nebulae by measuring the burst of radio waves. It can also brightness of stars within them. track severe rainfall by calculating The expanding universe the speed of precipitation. GPS As technology further improved, Hubble began to measure galaxy satellite navigation systems need spectrographic techniques were Doppler shifts, and by 1929 he had to account for the Doppler shift of applied to many other astronomical discovered a pattern: the further satellite signals in order to accurately objects, including mysterious, fuzzy a galaxy lies from Earth, the faster calculate a receiver unit’s position “nebulae.” Some of these proved to it moves away. This relationship, in relation to the satellites’ orbits. be huge clouds of interstellar gas, now known as Hubble’s law but They can also use this information emitting all their light at specific first predicted in 1922 by Russian to make precise measurements of wavelengths, similar to those physicist Alexander Friedmann, is the receiver unit’s own motion. ■ emitted by vapors in the laboratory. best interpreted as an effect of the But others emitted light in a universe expanding as a whole. continuum (a broad swathe of light of all colors) with a few dark lines, This so-called “cosmological” suggesting they were made up of redshift is not a result of the Doppler vast numbers of stars. These effect as Doppler would have became known as “spiral nebulae” understood it. Instead, expanding on account of their spiral shape. space drags galaxies apart, like raisins in a rising cake mix as it From 1912, American astronomer Vesto Slipher began to analyze the Named after Edwin Hubble, the spectra of distant spiral nebulae, Hubble Space Telescope has been used finding that the majority of them to discover distant stars, measure their had significant Doppler shifts to redshifts, and estimate the age of the the red end of the spectrum. This universe at 13.8 billion years. showed that they were moving away from Earth at high speeds, regardless of which part of the sky they were seen in. Some astronomers

192 IN CONTEXT MCWTHAYANESVSNTEEOESRTWISOEEUES KEY FIGURES Heinrich Hertz (1857–1894), ELECTROMAGNETIC WAVES Wilhelm Röntgen (1845–1923) BEFORE 1672 Isaac Newton splits white light into a spectrum using a prism and then recombines it. 1803 Thomas Young suggests that the colors of visible light are caused by rays of different wavelengths. AFTER c. 1894 Italian engineer Guglielmo Marconi achieves the first long-distance communication using radio waves. 1906 American inventor Reginald Fessenden uses an “amplitude modulation” system to make the first radio broadcast. I n 1865, James Clerk Maxwell interpreted light as a moving electromagnetic wave with transverse electric and magnetic fields—waves locked in step at right angles to each other (shown opposite). Maxwell’s theory caused scientists to ask further questions. How far did the “electromagnetic spectrum” extend beyond the range visible to the human eye? And what properties might distinguish waves with much longer or shorter wavelengths than visible light? Early discoveries German-born astronomer William Herschel found the first evidence for the existence of radiation beyond

SOUND AND LIGHT 193 See also: Force fields and Maxwell’s equations 142–147 ■ Lumpy and wavelike light 176–179 ■ Diffraction and interference 180–183 ■ Polarization 184–187 ■ Seeing beyond light 202–203 ■ Nuclear rays 238–239 Heinrich Hertz Born in Hamburg, Germany, in when light hits a material) 1857, Heinrich Hertz went on to and carried out important study sciences and engineering work investigating the way that in Dresden, Munich, and Berlin, forces are transferred between under eminent physicists such solid bodies in contact. Hertz’s as Gustav Kirchhoff and Hermann growing reputation led to an von Helmholtz. He obtained his appointment as director of the doctorate from the University of Physics Institute at Bonn in Berlin in 1880. 1889. Three years later, Hertz was diagnosed with a rare Hertz became a full professor disease of the blood vessels of physics at the University of and died in 1894. Karlsruhe in 1885. Here, he conducted groundbreaking Key works experiments to generate radio waves. He also contributed to 1893 Electric Waves the discovery of the photoelectric 1899 Principles of Mechanics effect (the emission of electrons the range of visible light in 1800. theories linked wavelength and waves with radically different While measuring temperatures color with electromagnetic waves, wavelengths. Most scientists associated with the different which meant that his model could concluded that the best way to (visible) colors in sunlight, also be applied to infrared and prove Maxwell’s model would Herschel allowed the spectrum ultraviolet rays (with wavelengths be to look for evidence of these of light he was projecting onto a shorter or longer than those within predicted phenomena. thermometer to drift beyond the the visible spectrum), allowing visible red light. He was surprised ultraviolet and infrared rays to be Heinrich Hertz, in 1886, to find the temperature reading had treated as natural extensions of experimented with an electrical shot up—a sign that much of the the visible spectrum. circuit that consisted of two heat in radiation from the sun is separate spiral-wound conducting carried by invisible rays. Finding proof wires placed near each other. Both Maxwell’s ideas remained ends of each wire terminated in a In 1801, German pharmacist theoretical, as at the time there metal ball, and when a current was Johann Ritter reported evidence were no technologies suitable to applied to one of the wires, a spark for what he called “chemical rays.” prove them. However, he was still would leap between the metal ball Ritter’s experiment involved able to predict phenomena that terminals of the other. The effect studying the behavior of silver would be associated with his model was an example of electromagnetic chloride, a light-sensitive chemical, of light—such as the existence of induction, with the spiral-wound which was relatively inactive when wires acting as “induction coils.” ❯❯ exposed to red light, but would Electromagnetic waves darken in blue light. Ritter showed are made up of two matching Magnetic field Direction of that exposing the chemical to waves at right angles—one Electric wave travel radiation beyond violet in the is an oscillating electric field, field visible spectrum (now known as and the other is an oscillating “ultraviolet” radiation) produced magnetic field. an even faster darkening reaction. Fields oscillate at Maxwell published his model right angles to of light as an electromagnetic wave each other that is self-reinforcing (supports itself continuously). Maxwell’s

194 ELECTROMAGNETIC WAVES Electromagnetic radiation Different types of flowed in the induction coil of the is a form of energy that electromagnetic radiation have circuit, it triggered sparks in the travels in waves. spark gap of that main circuit, and different wavelengths. also in the receiver’s spark gap. The receiver was well beyond the range Invisible electromagnetic Visible light is the only of any possible induction effects, so waves are longer or shorter form of electromagnetic something else had to be causing radiation that we can see. current to oscillate within it—the than light waves. electromagnetic waves. Current flowing in one coiled that oscillated rapidly back and Hertz carried out a variety of wire generated a magnetic field forth. With careful adjustments of further tests to prove that he had that caused current to flow in the the currents and voltages, Hertz indeed produced electromagnetic other—but as Hertz studied this was able to “tune” his circuit to waves similar to light—such as experiment more closely, he an oscillation frequency of around showing that the waves traveled at formulated an idea for a circuit 50 million cycles per second. the speed of light. He published his that could test Maxwell’s theories. results widely. According to Maxwell’s theory, Hertz’s circuit, completed in this oscillating current would Tuning into radio 1888, had a pair of long wires produce electromagnetic waves Other physicists and inventors running toward each other with with wavelengths of a few feet, soon began investigating these a tiny “spark gap” between their which could be detected at a “Hertzian waves” (later called ends. The other end of each wire distance. The final element of radio waves) and found countless was attached to its own 12-inch Hertz’s experiment was a “receiver” applications. As the technology (30-cm) zinc sphere. Running (which would receive the wave improved so did the range and current through an “induction coil” signals)—this was a separate quality of signals, and the ability to nearby induced sparks across the rectangle of copper wire with its broadcast many different streams spark gap, creating a high voltage own spark gap, mounted at some of radio waves from a single mast. difference between the two ends of distance from the main circuit. Wireless telegraphy (simple signals each wire and an electric current Hertz found that when current transmitted as Morse code) was followed by voice communication and eventually television. Hertz’s radio waves still play a vital role in modern-day technology. Radio waves The electromagnetic spectrum Infrared (1 km–10 cm) (100 μm–c. 740 nm) Microwaves (1 cm–1 mm) Dish antennas Microwave ovens A remote can capture radio heat up food by using control sends waves to help microwaves to make signals to a astronomers the water molecules television using detect stars. within the food vibrate. infrared waves.

SOUND AND LIGHT 195 By the time radio communications The first X-ray image ever taken was overexposure on living tissue were were becoming a reality, another made by the silhouette of Anna Bertha noticed, and steps were gradually different type of electromagnetic Röntgen’s hand on a photographic introduced to limit exposure. This radiation had been discovered. In plate, in 1895. Her wedding ring is also included a return to X-ray 1895, German engineer Wilhelm visible on her fourth finger. photographs that required only a Röntgen was experimenting with brief burst of rays. The true nature the properties of cathode rays. detector screen was glowing of X-rays was debated until 1912 These are streams of electrons during his experiments, and this when Max von Laue used crystals observed in vacuum tubes, which was caused by unknown rays to successfully diffract X-rays are emitted from a cathode (the being emitted from inside the (diffraction occurs when any wave electrode connected to the negative tube, which were passing straight encounters an obstacle). This terminal of a voltage supply) and through the cardboard casing. proved that X-rays were waves then released inside a discharge and a high-energy form of tube (a glass vessel with a very X-ray imaging electromagnetic radiation. ■ high voltage between its ends). To Tests revealed that Röntgen’s new avoid any possible effects of light rays (which he called “X” rays, on the tube, Röntgen wrapped it indicating their unknown nature) in cardboard. A nearby fluorescent could also affect photographic film, meaning that he could permanently I have seen record their behavior. He carried my death! out experiments to test what types Anna Röntgen of material the rays would pass through and discovered that they on seeing the first X-ray image were blocked by metal. Röntgen revealing the bones of her hand asked his wife, Anna, to place her hand on a photographic plate while he directed the X-rays at it; he found that bones blocked the rays but soft tissue did not. Scientists and inventors raced to develop new applications for X-ray imaging. In the early 1900s, the damaging effects of X-ray Visible Ultraviolet X-rays Gamma rays (c. 740 nm–380 nm) (380 nm–10 nm) (10 nm–0.01 nm) (0.01 nm–0.00001 nm) The human eye Disinfection can be X-rays pass Nuclear power can only see this carried out by using through tissue stations use the short range of the some wavelengths to reveal the energy of gamma electromagnetic of UV light to teeth or bones radiation to generate spectrum. kill bacteria. underneath. electricity.

196 IN CONTEXT IOOTSHFFAESTTHLPRAEECUNSETGPRUMHAAEUGRSEEICS KEY FIGURE Niels Bohr (1885–1962) LIGHT FROM THE ATOM BEFORE 1565 In Spain, Nicolás Monardes describes the fluorescent properties of a kidney wood infusion. 1669 Hennig Brand, a German chemist, discovers phosphorus, which glows in the dark after being illuminated. AFTER 1926 Austrian physicist Erwin Schrödinger shows that electron orbits resemble fuzzy clouds rather than circular paths. 1953 In the US, Charles Townes uses stimulated emission from electron movements to create a microwave amplifier, demonstrating the principle that foreshadows the laser. T he ability of materials to generate light rather than simply reflect it from luminous sources such as the sun was initially regarded with mild curiosity. Eventually, however, it proved essential to understanding the atomic structure of matter, and in the 20th century it led to valuable new technologies. Discovering fluorescence Substances with a natural ability to glow in certain conditions were recorded from at least the 16th century, but scientists did not attempt to investigate this phenomenon until the early 1800s. In 1819, English clergyman and

SOUND AND LIGHT 197 See also: Electric potential 128–129 ■ Electromagnetic waves 192–195 ■ Energy quanta 208–211 ■ Particles and waves 212–215 ■ Matrices and waves 218–219 ■ The nucleus 240–241 ■ Subatomic particles 242–243 Some minerals are fluorescent. Like chemical “fingerprint”. In 1860 this fluorite, they absorb certain types and 1861, Bunsen and Kirchhoff of light, such as ultraviolet, and then identified two new elements— release it at a different wavelength, cesium and rubidium—from changing its perceived color. their emission lines alone. mineralogist Edward Clarke Michael Faraday, Stokes explained Balmer’s breakthrough described the properties of the the phenomenon in terms of gas Despite the success of Kirchhoff mineral fluorspar (now better atoms becoming energized, and Bunsen’s method, they had known as fluorite), which glows allowing current to flow through yet to explain why the emission in certain circumstances. them, then releasing light as they lines were produced at specific shed their energy. wavelengths. However, they In 1852, Irish physicist George realized it had something to do Gabriel Stokes showed that the A few years later, German with the properties of atoms in mineral’s glow was caused by glassblower Heinrich Geissler individual elements. This was a exposure to ultraviolet light, while invented a new means of creating new concept. At the time, atoms the emitted light was limited to a much better vacuums in glass were generally thought of as single blue color and wavelength, vessels. By adding specific gases solid, indivisible particles of a appearing as a line when it was to a vacuum, he found he could particular element, so it was split apart into a spectrum. He cause a lamp to glow in various difficult to imagine an internal argued that fluorspar transformed colors. This discovery lies behind process that generated light or shorter-wavelength ultraviolet the invention of fluorescent lights, altered the wavelengths of the light directly into visible light which became widespread in the emission lines. by somehow stretching its 20th century. wavelength, and coined the term An important breakthrough “fluorescence” to describe this Elementary emissions came in 1885, when Swiss particular behavior. The reason why some atoms mathematician Johann Jakob produce light remained a mystery Balmer identified a pattern in the Meanwhile, early electrical until the late 1850s, when fellow series of emission lines created experimenters had discovered Germans chemist Robert Bunsen by hydrogen. The wavelengths, another means of creating luminous and physicist Gustav Kirchhoff which until then had seemed matter. When they attempted to joined forces to investigate the like a random collection of pass electric current between metal phenomenon. They focused on lines, could be predicted by a electrodes at either end of a glass the colors generated when mathematical formula involving ❯❯ tube that had as much air removed different elements were heated to as possible, the thin gas that incandescent temperatures in the A chemist who is remained between the electrodes searing flame of Bunsen’s recently not a physicist began to glow when the voltage invented laboratory gas burner. is nothing at all. difference between the electrodes They found that the light produced was sufficiently high. Working with was not a blended continuum Robert Bunsen of different wavelengths and colors—like sunlight or the light from stars—but was a mix of a few bright lines emitted with very specific wavelengths and colors (emission lines). The precise pattern of emissions was different for each element and was therefore a unique

198 LIGHT FROM THE ATOM We have an intimate the atom with a tiny central sometimes produce light in small knowledge of the constituents nucleus surrounded by electrons bursts of specific wavelength scattered at random throughout and energy. of … individual atoms. the rest of the volume. Niels Bohr Bohr’s model Discussions with Rutherford In 1913, Bohr found a means of two series of whole numbers. inspired a young Danish scientist, linking atomic structure to the Emission lines linked with higher Niels Bohr, to work on his own Rydberg formula for the first time. values of either or both of these atomic model. Bohr’s breakthrough In an influential trilogy of papers, numbers were only generated in lay in combining Rutherford’s he suggested that the movement higher-temperature environments. nuclear model with Max Planck’s of electrons in an atom was radical suggestion, in 1900, that, constrained so that they could Balmer’s formula, called the in particular circumstances, only have certain values of angular Balmer series, immediately proved radiation was emitted in bite-size momentum (the momentum due its worth when the high-energy chunks called quanta. Planck to their orbit around the nucleus). absorption lines associated with had suggested this theory as a In practice this meant that hydrogen were identified at the mathematical means of explaining electrons could only orbit at certain predicted wavelengths in the the characteristic light output of fixed distances from the nucleus. spectrum of the sun and other stars and other incandescent Because the strength of the stars. In 1888, Swedish physicist objects. In 1905, Albert Einstein electromagnetic attraction Johannes Rydberg developed a had gone a step further than Planck, between the positively charged more generalized version of the suggesting that these small bursts nucleus and the negatively formula, subsequently called the of electromagnetic radiation were charged electrons would also vary, Rydberg formula, which could not merely a consequence of certain depending on the electron’s orbit, be used (with a few tweaks) to types of radiation emission, but were each electron could be said to have predict emission lines from many fundamental to the nature of light. a certain energy, with lower-energy different elements. orbits close to the nucleus and Bohr did not, at this point in higher-energy ones further out. Clues from the atom time, accept Einstein’s theory that The 1890s and the beginning of the light always traveled in bursts Each orbit could hold a 20th century saw major advances called photons, but he did wonder maximum number of electrons, in the understanding of atoms. whether something about atomic and orbits close to the nucleus Evidence emerged that atoms were structure, and in particular the “filled up” first. Empty spaces not the solid, uniform lumps of arrangement of electrons, could matter previously assumed. First, in 1897, J.J. Thomson discovered An atom consists of a nucleus and electrons, which the electron (the first subatomic circle the nucleus at different distances from it. particle), and then in 1909, building on the work of Hans Geiger If an electron moves The further an and Ernest Marsden, Ernest closer to the nucleus, it electron is from the Rutherford discovered the nucleus, loses energy, which is nucleus, the more in which most of an atom’s mass is concentrated. Rutherford imagined emitted as light. energy it has. Atoms can emit light.