SCIENCE

ctivit To show the atmospheric pressure Take a glass filled with water and cover its mouth with a cardboard. Invert the glass supporting the cardboard with your palm. Hold the glass in your hand and remove the other hand slowly which was supporting the cardboard. The atmosphere exerts pressure upwards on the cardboard. Thus, the cardboard and the water do not fall down. This proves that atmosphere exerts pressure from all directions. Properties of air Air is a distinct kind of matter having no fixed shape and volume. But it has the following properties. i) Air can occupy space. ii) Air has weight. iii) Air exerts pressure. (i) Air occupies space ctivit To show that air occupies apace Take a water trough filled with water. Dip an empty glass tumbler into the trough holding it upside straight down. The water doesn’t enter the glass. Now, slowly fill the glass. Water enters the glass whereas air bubbles come out of it. It means at first, the glass was filled with air though it looked empty. It was then displaced by water and air bubbles came out of it. This concludes that air occupies space. (ii) Air has weight ctivit To show that air has weight Weigh two identical footballs, one inflated and the other flat by using a physical balance. We find that the inflated football has more weight than the flat one. This proves that air has weight and air can be compressed. The inflated football is easier to move than the flat one due to the air pressure inside it. Thus, the wheels of the vehicle are also filled with air. Pressure 47

(iii) Air exerts pressure ctivit To show air exerts pressure. Take some water in a bottle. Suck it by using a straw pipe. When the air from the pipe is sucked up, the water enters the mouth through the straw pipe. It is because when the air from the pipe is sucked out, the air pressure inside the pipe becomes less than the outside pressure. Hence the water enters the straw and comes into the mouth due to the more atmospheric pressure at the surface of the liquid. This also proves that atmosphere exerts pressure. We fill ink in a pen and doctors fill medicine in the syringe on the basis of the same principle. Application of atmospheric pressure Atmospheric pressure is very important in our daily life. It is used to make different machines and apparatuses. It is used in the following activities. › It is used to fill medicine in a syringe. › It is used to fill ink in a pen. › To run a air filling machines. › To make a water pumping machine. › To use a vacuum cleaner. Questions i. How is a pen filled with ink? ii. How can you prove that atmosphere exerts pressure? Liquid pressure Like air, liquid also exerts pressure. The pressure exerted by liquid on the walls and bottom of the vessel in which it is kept is called liquid pressure. Let’s do the following activities to prove that liquid also exerts pressure. ctivit To prove that liquid applies pressure Take a long transparent glass tube open at both the ends and a rubber balloon. Tie the balloon at one end of the pipe and hold it at the middle keeping it in a vertical position as shown in the figure. Pour some water into the pipe. Does the rubber balloon bulge out? Also note the height of the water column in the pipe. Again pour some more water and observe the bulging in the balloon. Is there any change? 48 New Creative Science and Environment Book - 8

ater eve after Water level before Balloon before Ba n after The rubber balloon bulges out as we pour water into the tube. It is due to the pressure exerted by water at the bottom of the vessel. This shows that liquid exerts pressure at the bottom of the vessel, and the pressure increases at the depth as the bulging of the balloon increases when we pour more water into it. Properties of liquid Liquids have their fixed volume and weight. Hence, they also exert pressure as mentioned above. Liquids have certain properties of their own, which can be described below. i) Liquid pressure increases with its depth. ii) Liquid finds its own level. iii) Liquid transmits pressure equally in all the directions. iv) The liquid pressure at any point depends on its density. Liquid pressure increases with its depth A simple activity can prove that liquid pressure increases with depth. ctivit To prove that liquid pressure depends upon the depth Take a plastic vessel with three holes of the same size at the same side at different levels as shown in the figure. Fill the vessel with water and observe it. Does the water come out with an equal force from all the holes? You will find that the water comes out with more force from the lowest hole (c) than from other holes. It shows that liquid pressure increases with the increase in depth from its free surface. Due to the more liquid pressure at the bottom, the dams are made wider at their bases, than at their tops. Deep sea-divers also wear special diving suits due to this. Pressure 49

Questions i. Why are dams made wider at the bases? ii. Deep sea-divers wear special diving suits. Why? Liquid finds its own level ctivit To show that liquid makes its own level Take a set of communicating vessels (Pascal’s tube) as shown in the figure. Pour water into one of them and observe the water level in all. Is the water level equal in all of these vessels? The water level is equal in all of these vessels. It is because liquid finds its own level. Liquid transmits pressure equally in all directions ctivit To show the transmission of liquid pressure Take a plastic bottle and make holes in its all faces. Fill it with water and tighten the lid. Now, press the bottle. What do you see? The water comes out equally from each hole. This proves that, when pressure is applied to an enclosed liquid, it transmits pressure equally in all directions. It is also known as Pascal’s law. Memory Note ; Pascal’s law was formulated by a French scientist Blaise Pascal in 1647 AD. The liquid pressure at any point depends on its density It is found that the pressure exerted by the mercury level is more than at the same height of water. It is because of the more density of mercury than water. It proves that liquid pressure increases with the density of the liquid. 50 New Creative Science and Environment Book - 8

Measurement of liquid pressure Consider a liquid of density (d) is kept in a container having the base area (A) up to the depth (h) from the free surface of the liquid as shown in the figure. We know that, Pressure (P) = Force (F) [where F = mg, m= mass of liquid Area (A) and g = acceleration due to gravity] = M× g [m = d × v, where v is the volume d A h of liquid] = d×v×g [Where, V = A × h] A A = d×A×h×g A ∴ P =d×g×h This is the expression for the pressure exerted by liquid. It shows that liquid pressure depends on three factors. They are: a) Depth of the liquid from its free surface (h) b) Density of the liquid (d) c) Acceleration due to gravity (g) For a liquid at a given place, its density and acceleration due to gravity are constant. So, liquid pressure (P) ∝ depth of the liquid from the free surface (h). Density In different substances, there are different numbers of molecules. Different molecules have mass. Thus, we feel difficulty to lift different substances of the same volume. This is due to density of the substance. It is basically a measurement of how tightly matter is crowded together. Density is defined as the amount of mass present in per unit volume. The principle of density was discovered by the Greek scientist Archimedes. Mathematically, Density (d) = m v The SI unit of density is kilogram per cubic meter (kg/m3). It is also frequently denoted in the CGS unit of gram per cubic centimetre (g/cm3). The density of water is 1000 kg/m3. The matter with high density has more mass than the matter with less density if the volume of both the matter is the same. Pressure 51

Relative Density (or Specific Gravity) Relative density or specific gravity is a ratio of density between the substance and pure water at 4 0C. Thus, relative density is the ratio expressing the weight of a given material compared to that of an equal volume of water at 40C. The ratio of the densities of two substances is called the relative density or specific gravity. If water is taken as the reference material, relative density is defined as the ratio of the density of a substance to the density of water at 40 C at standard atmospheric pressure. I.e. Relative density = Density of substance Density of water at 4°C If the relative density or the specific gravity of gold is 19.2 then it means that the density of gold is 19.2 times the density of water at 40C. Floating and Sinking of object We can observe that some substances like paper, cork, wood, plastics, leaf of plants, etc. float on water while some other substances sink. For example, a wooden plank floats but a similar sheet of iron sinks. How does it happen and how can we say whether the substance will sink or float in water? To answer the above question, we should have Floating woods and sinking of iron nails the idea of density. The density of the given substance and the density of the liquid are the determining factors for the floating and sinking of the object. If the density of an object is less than the density of the liquid, it floats. If the density of an object is greater than the density of the liquid it sinks. In other words, the substance whose relative density with the liquid is less then it sinks in the liquid while the substance with relative density floats. Now, if you see any substance floating in water, then you can immediately say that the density of the substance is less than the density of water. And if the substance is sinking in water, then the density of the substance is greater than the density of water. ctivit Collect any 10 different objects found in your surroundings and check which one is sinking and which one is floating on water. Fill the given table. S.N. Substances with density more Substances with density less than the than the density of the water density of the water 52 New Creative Science and Environment Book - 8

Solved Numericals 1. The dimension of a certain tank is 3 m × 2 m × 2 m. What is the pressure at the bottom of the tank if the tank is half filled? (Given, density of water = 1000 kg/ m3 and g = 10 m/s2) Solution: Given, density (d) = 1000 kg/m3 Depth (h) = 1m Acceleration due to gravity (g) = 10 m/s2 Water pressure (P) =? We know that, P =h×d×g = 1 × 1000 × 10 = 10000 Pascal ∴ The pressure exerted by water is 10,000 Pascal. 2. A liquid exerts the pressure of 5000 Pascal at the bottom of the vessel. If the depth of the liquid is 2 m, find its density. Solution: Given, pressure (P) = 5000 Pa Depth (h) =2m Density (d) =? Acceleration due to gravity (g) = 10 m/s2 We know that, P =h×d×g or, 5000 = 2 × d × 10 or, d = 5000 ∴d 20 = 250 kg/m3 Hence, the density of the liquid is 250 kg/m3 3. Find the density of an object if its mass is 8000 kg and volume is 1000 m3. Solution: Given that, Mass (m) = 8000 kg Volume (v) Density (d) = 1000 m3 Density (d) =? Density (d) = m Density (d) = 8V000 1000 = 8 kg/m3 Pressure 53

Answer writing skill 1. All cutting instruments have the sharp edges. Why?  The sharp edge makes the cross-sectional area of the cutting instruments small. As F we know, P = A So, the applied force will get distributed over small areas thereby increasing pressure. This makes the cutting easier. Therefore, cutting instruments are made with sharp edges. 2. A girl with high-heeled shoes makes much more depression on the sand than an elephant. Why?  The high-heeled shoes of the girl has less area. As we know, P = F A So, the pressure exerted by the girl will be more on the sand than that of an elephant. Hence, she will exert more depression than the elephant. 3. Dams are made wider at the base. Why?  As we know liquid pressure increases with depth. So, to prevent the dams from collapsing due to the enormous liquid pressure, they are made wider at the base. 4. Deep sea-divers wear diving suits. Why?  As we know, liquid pressure (P) = h × d × g It shows that the liquid pressure increases with depth from its free surface. So, to withstand the enormous pressure applied by water, the deep sea-divers wear special diving suits. 5. We feel uneasy at high altitude. Why?  As the altitude increases, the atmospheric pressure decreases and hence our internal blood pressure increases. Due to this, we may have altitude sickness and the nose or ear bleed. So, we feel uneasy. 6. The mercury level decreases in the barometer as the altitude increases. Why?  The barometer is used to measure the atmospheric pressure of a certain place on the earth’s surface. As the altitude increases, the atmospheric pressure decreases. So, the level of the mercury decreases in the barometer. 7. Atmosphere exerts enormous pressure to our body but we do not feel it, why?  Our internal body pressure is nearly equal to the outward pressure. This balances blood pressure and atmospheric pressure. Thus, we do not feel the enormous pressure of the atmosphere. 54 New Creative Science and Environment Book - 8

SUM M ARY ” The force acting perpendicularly on per unit surface area is called pressure. Its SI unit is N/m2 or Pascal. ” The pressure exerted by atmosphere is called atmospheric pressure. ” The normal atmospheric pressure i.e. atmospheric pressure at the sea level is 760 mm of Hg. ” Air has weight and it can be easily compressed. ” Air occupies space. ” Atmospheric pressure decreases with the increase in altitude. ” The pressure exerted by liquid is called liquid pressure. ” Liquid exerts pressure at the bottom of the vessel as well as on the sides of the vessel in which it is kept. ” Liquid pressure increases with the increase in depth. ” Liquid exerts pressure equally in all directions. ” When the pressure is applied to an enclosed liquid, then it transmits pressure equally in all directions. This is called the Pascal’s law. ” Liquid pressure increases with the increase in density of the liquids. Exercise 1. Fill in the blanks. (a) The relation of pressure, force and area is ___________. (b) Atmospheric pressure ____________ as we go up from the surface of the earth. (c) Liquid finds its own ____________. (d) The value of the atmospheric pressure at the sea level is ____________. 2. Choose the best answer. (a) The SI unit of pressure is: (i) Nm2 (ii) Nm (iii) Nm–2 (iv) N (b) The pressure exerted by water of depth 100 m is (Take g = 10 m/s2) (i) 102 Nm–2 (ii) 10 Nm–2 (iii) 106 Nm–2 (iv) 104 Nm–2 Pressure 55

(c) The instrument used to measure the atmospheric pressure is: (i) Barometer (ii) Pressure gauge (iii) Manometer (iv) Pressure meter (d) Liquid exerts pressure: (i) Only in upward direction (ii) Only in downward direction (iii) In one side only (iv) In all direction (e) _____________ is the barometric substance. (i) Water (ii) Mercury (iii) Air (iv) Vapour 3. Write the differences between: (a) Force and pressure (b) Liquid pressure and air pressure 4. Give reasons. (a) It is easier to cut with sharp instruments than with blunt ones. (b) Foundations of buildings are made wider. (c) Dams are made wider at the bases. (d) The level of mercury falls as the altitude increases. (e) Water is not used as the barometric substance. (f) Heavy vehicles have back wheels in pairs. (g) The balloon bursts finally when we go on filling air in it. 5. Answer the following questions. (a) Define pressure. Write its SI unit. (b) Write the experiment to prove, (i) Atmosphere exerts pressure (ii) Liquid pressure increases with the increase in depth (iii) Pascal’s law (c) How can we fill ink inside a pen? (d) Prove P = h×d×g (e) Define density, relative density, sinking and floating. (f) The atmosphere exerts enormous pressure to us. But we do not get crushed. Why? (g) Nose bleeding may occur at a high altitude. Why? 56 New Creative Science and Environment Book - 8

Numerical Problems 1. The weight of a boy is 200 N and his base area is 2 m2. Calculate the pressure exerted by him. (Ans: 100 Pa) 2. Find the pressure exerted by a liquid of depth of 8 m. Take g = 10 ms–2 and density of liquid = 1000 kg/m3. (Ans: 8 × 104 Pa) 3. If a tank has water to the depth of 10 m, calculate the pressure exerted by it. (Ans: 105 Pa) 4. A box has 300 kg mass. The surface area of the box is 150 m2. Find the pressure exerted by the box on the floor. (Ans: 20 Pa) 5. The pressure exerted by a liquid column of depth 5 m on the base of the vessel is 5000 Nm–2. Find the density of that liquid. Take g = 10 m/s2. (Ans: 100 kg/m3) 6. Find the density of an object if its mass is 18000 kg and volume is 1000 m3. (Ans: 18 kg) 7. Find the density of an object if its mass is 16000 g and volume is 1000 cm3. (Ans: 16 g) lossar – an instrument for showing the height above the sea level – marked units of measurement A eter – the height above the sea level C alibrated - applied A tude Ex erted Pressure 57

U5 WORK, ENERGY AND POWER L earning O utc om es At the end of this unit, students will be able to: ~ say the relation and differences among energy, work and power. ~ explain the simple transformation of energy and demonstrate it. ~ write the formula of energy, work and power and solve simple numerical problems. Main points to be focused ~ Forms of energy ~ Work ~ Law of conservation of energy ~ Work against gravity ~ Power ~ Work done against friction ~ Energy Introduction The concept of work is closely related with the concept of energy. In fact, the concept of work provides a link between force and energy. The general idea of work and energy can be applied to a wide range of phenomena in different fields of physics. Further from the practical point of view, it is important to know not only the work done to an object but also the rate at which it is being done. This involves the concept of power. We shall discuss these concepts in this chapter in detail. Work The traditional meaning of work is quite different from its scientific meaning. In our everyday activity, the term ‘work’ is used equally for mental and physical work. As we say the watchman is continuously standing at the gate and has become completely tired. So, he has done work. Similarly a student is preparing for his exam by 58 New Creative Science and Environment Book - 8

continuously reading and practicing questions. So, we can say that she is working hard. But in scientific concept, both of them have not done any work because they have not changed their position in spite of the expenses of their force. In physics, “work is said to be done if anybody changes its position by the application of force on it. Thus, for the work to be done, i) A force should be applied to the object and ii) The object should displace from its original position. Some examples of work are lifting an object from the floor and putting it on the table, moving of cart due to the pulling of a bullock, pushing of a van, etc. Mathematically, work is the product of force and displacement, i.e. work (W) = Force (F) × Displacement (S) ∴ W=FxS Its SI unit is joule or (Nm). From the above expression, it is clear that, When force = 1 N, displacement = 1 m, then Work = 1 joule. Hence, when a body covers the displacement of 1 m by the application of 1 N force, then the work done is said to be 1 joule work. Work done by a constant force The direction of the displacement of an object and the direction of the force can have different relations with each other. The directions of the force and the displacement may be: i) Same ii) Opposite iii) Perpendicular to each other i) Force acting in the same direction of displacement If the displacement of a body is the same as the direction of the force, then the work done by the body is positive, e.g. a body falling freely under the effect of gravity. Work, energy and power 59

ii) Force acting in the opposite direction of displacement If the displacement of the body is in the opposite direction of the force then the work done by the body is negative, e.g. a ball thrown upwards against gravity, work done by frictional force, etc. iii) Force acting in the perpendicular direction of displacement If the displacement of the body is perpendicular to the direction of force then no work is said to be done, as there is no displacement in the direction to the force, e.g. when a block is moving around a circular path, the work done by centripetal force is zero. Memory Note ; The unit of work and energy i.e. joule is named after the British physicist James Prescott Joule. Questions i. State a situation in which force is applied to a body but no work is done? ii. A person standing for a whole day feels tired even if he does not seem to do any work. Why? Types of work There are two types of work done. They are: i) Work against gravity ii) Work against friction i) Work against gravity If the displacement of a body is opposite to the direction of the gravitational force of the earth then the work is called work against gravity, e.g. lifting a load from the ground by applying some force. Solved Numericals Calculate the work done against gravity when 3 kg stone is raised by 3 m. Solution: Work done against gravity = force × displacement 60 New Creative Science and Environment Book - 8

= mass × acceleration due to gravity × height [ Force = mass × acceleration due to gravity] ∴ W =m×g×h = 3 kg × 10 ms–2 × 3 m = 90 joule Hence, the work done against gravity is 90 joule. ii) Work done against friction If the displacement of the body is opposite to the direction of the frictional force then the work is called work against friction, e.g. pushing a car to a certain distance, pulling a wooden plank to a certain distance. Memory Note The acceleration produced on a body due to the gravity of the earth is called acceleration due to gravity. Its average value is 9.8 m/s2. Here, for our convenience, we take g = 10 m/s2 Force = mass × g [From Newton’s 2nd law of motion] Solved Numericals 1. Find the work done by a boy if he applies the force of 5 N to displace a book through 20 cm. Solution: Given, force (F) = 5 N Displacement (S) = 20 cm Work (W) = 20 = 0.2 m 100 =? We have, W =F×d = 5 × 0.2 = 1 joule ∴ The work done by the boy is 1 j. 2. Calculate work done against friction when 5 kg mass is dragged about 5 m distance. ∴ Work done against friction = Force × displacement = mass × g × displacement ( F = m×g) = 5 × 10 × 5 = 250 joule Hence, the work done against friction is 250 joule. Work, energy and power 61

Energy We use energy to do work and make all the movements. When we eat, our body transforms the food into energy to do work. When we run or walk or do some physical work, we use energy. Cars, planes, trains and various machines transform energy into work. In general, anybody able to perform work is said to possess energy. Thus, the energy of an object is defined as the capacity for doing work. Its SI unit is joule. One joule energy is the energy required to do 1 joule work. Bigger units of energy are, 1 kj = 1000 j 1 mj = 106 j Forms of energy There are various forms of energy. They are: 1) Mechanical energy 5) Magnetic energy 2) Heat energy 6) Electrical energy 3) Light energy 7) Chemical energy 4) Sound energy 8) Nuclear energy 1) Mechanical energy The energy possessed by the state of the body (either rest or motion state) is called the mechanical energy. It again has two forms. They are: (a) Kinetic energy (b) Potential energy (a) Kinetic energy The energy possessed by a body by the virtue of its motion is called the kinetic energy. Mathematically, the kinetic energy of a moving object is defined as half the product of its mass (m) and the square of the velocity of that object (v). Therefore, kinetic energy (K.E) = 1 mass × (velocity)2 2 ∴ 1 K.E. = 2 mv2 62 New Creative Science and Environment Book - 8

Examples of bodies possessing the kinetic energy are: i) A ball rolling on a surface ii) A bullet fired from a gun iii) Water in motion iv) A falling body v) Moving car vi) A running man, etc. (b) Potential energy The energy possessed by a body by the virtue of its position is called the potential energy. Mathematically, potential energy of a body is defined as the product of its mass (m), acceleration due to gravity (g) and its height from the ground (h). Therefore, Earth Potential energy (P.E) = mass × acceleration due to gravity × height ∴ P.E. = mgh The potential energy may be gravitational or elastic. Examples of bodies possessing the potential energy are: i) A stretched rubber of catapult ii) A stretched bow iii) Water stored in a dam iv) A stone at a height, etc. 2) Heat energy Heat is a form of energy produced due to the random vibration of the molecules of the body. It flows from the body at a higher temperature to a body at a lower temperature. It gives us the sensation of the warmth. The heat energy is used to do various types of work such as cooking food, making our body warm, boiling water, running engines, etc. It is also produced due to the combustion of coal, petrol, mineral oil, chemical reaction, nuclear reaction, etc. Memory Note ; The sun gives us enormous amount of heat and light. The energy in the sun is produced due to the nuclear reaction in which hydrogen atoms are continuously changing into helium atoms. Work, energy and power 63

3) Light energy Light is a form of energy which gives us the sensation of vision. We get light from the sun, a burning candle, a glowing bulb, etc. The sunlight trapped by chlorophyll is used by green plants to prepare their food. So, due to the sunlight, all living things can exist on the earth. 4) Sound energy Sound is a form of energy produced due to the vibration of the molecules of a body. It gives us the sensation of hearing to our ears. It can produce disturbance in a medium, through which it propagates. So, it is also regarded as a form of energy. 5) Magnetic energy The energy possessed by a magnet with the help of which it can show the attractive and directive property is called the magnetic energy. With the help of the magnetic energy, a magnet can attract magnetic substances like iron, nickel, cobalt, etc. So, the magnetic energy is used to separate iron dust from soil, iron dust from eyes and in factories, it is also used to lift heavy loads. 6) Electrical energy It is a form of energy produced due to the change in number of electrons in a body or flow of electrons through a conductor. Electricity is the most versatile form of energy that can be easily changed into other forms like heat, sound, light, etc. Now a-days electricity is used to run electrical appliances like TV, radio, refrigerator, bulbs, vehicles, cookers, etc. 7) Chemical energy The energy released by a body when it undergoes chemical change or reaction is called the chemical energy. Our food consists of chemical energy, when it gets oxidized with air then it gets converted into heat energy. It is used to perform various activities of our body. Similarly, the chemical energy stored in fuel when burns with oxygen, releases in the form of heat. It is used to run vehicles. 64 New Creative Science and Environment Book - 8

8) Nuclear energy The tremendous amount of energy released by changing the nucleus of one atom into another is called the nuclear energy. The heavy atoms are unstable and they break into simpler nuclei. This process is called the nuclear fission. Similarly the lighter nuclei get combined and form a heavier nucleus and the process is called nuclear fusion. In both of these processes, enormous energy is released and this energy is called the nuclear energy. Atom bombs and hydrogen bombs are based on the nuclear energy. The nuclear energy is also used to generate electricity. Law of conservation of energy It states that, “energy can neither be created nor be destroyed but can be changed from one form to another’’. Memory Note Lavoisier in 1774 stated that matter can neither be created nor be destroyed. This was called the law of conservation of mass. But according to Albert Einstein, mass can be converted into energy and vice-versa. Thus, the laws of conservation of mass and energy have become one, which is called the law of conservation of mass-energy. ctivit Observe a nearby mill such as a rice mill, oil mill, wind mill or water mill and observe how the energy is being transformed in them. Power All of us cannot do the same work at the same time. For example, a stronger man may do a certain types of work in a relatively less time than a weaker person. Similarly, a more powerful vehicle can complete a journey in a shorter time than a less powerful one. Thus, power measures the speed of work done. Therefore, the rate of doing work is called power. Mathematically, Power (P) = Work done (w) Time taken (t) ∴ P = w t Its SI unit is js–1 or watt. 1 watt power (1 js–1 power) : Work, energy and power 65

If 1 joule work is done in one second then the power is called one watt power. The bigger units of power are kilowatt (kw), megawatt (mw) and horsepower (hp) 1000 watt = 1 kw 106 watt = 1 mw 746 watt = 1 hp The power of engine is measured in horsepower. Solved Numericals 1. Find the power of a crane if it lifts a load of 750 N up to the height of 200 m in 100 seconds. Solution: Given, Force (F) = 750 N Height (h) = 200 m Time taken (t) = 100s Power (P) =? We have, P = w [ W = F × d] or, P = ft× d ∴ P = 1500 watt or, P = 75t0 × 200 100 Hence, the power of the crane is 1500 watt. 2. A boy of mass 50 kg runs up a staircase of 45 steps in 9 seconds. If the height of each step is 15 cm, find his power. Take g = 10 ms–2. Solution: Given, Weight of the boy (w) = mg = 50 × 10 = 500 N Height of the staircase (h) = 45 × 15 m = 6.75 m 100 Time taken (t) = 9s We have, = Work done (w) Power (P) time taken (t) = force × distance time taken = m × g × h t = 500 × 6.75 = 375 w 9 ∴ His power is 375 w. 66 New Creative Science and Environment Book - 8

Answer writing skills 1. A boy standing for the whole day feels tired. Has he done any work? Why does he feel tired?  A boy standing for the whole day has not done any work because he hasn’t changed his position but he feels tired because he is applying force against the force of gravity. 2. State the conditions under which a force does no work.  A force does not work, when i) the displacement is zero. ii) the displacement is perpendicular to the direction of force. 3. Can the kinetic energy of the body be negative?  No, because, K.E. = 1 mv2. Mass and velocity would never have negative values. 2 4. Prove heat is a form of energy.  Heat is used to warm water and the steam so generated is used to run steam engines. Thus, heat has the capacity of doing work, and hence it is a form of energy. 5. An electric bulb is marked 60 watt. What do you mean by it?  It means that the bulb can convert 60 Jules of electric energy into heat and light energy in 1 second. SUM M ARY ” Work is said to be done if anybody changes its position by the application of force it. ” The SI unit of work is joule. ” The work may be i) work against gravity, and ii) work against friction ” Energy is the capacity of doing work. Its SI unit is joule. ” 1 joule energy is the energy capable of doing 1 joule work. ” The forms of energy are: a) Mechanical energy b) Light energy c) Heat energy d) Sound energy e) Electrical energy f) Magnetic energy g) Chemical energy h) Nuclear energy ” Mechanical energy is of two types a) Kinetic energy b) Potential energy. Work, energy and power 67

” We get energy in different ways like: a) heat and light energy from the sun. b) kinetic energy from wind and running water. c) magnetic energy from a magnet, etc. ” The rate of doing work is called power. Its SI unit is watt. Exercise 1. Fill in the blanks. (a) The relation of work, force and displacement is ___________. (b) The SI unit of work is ___________. (c) ___________ is the capacity of doing work. (d) The rate of doing work is called __________. (e) A running man has ___________ energy. 2. Write 'T' for true and 'F' for false statements. (a) When a person stands for the whole day, some work is said to be done by him. (b) The SI unit of power is joule. (c) A stone at a greater height has potential energy. (d) Heat has not capacity of doing work. (e) A body having more power can do the work in less time. 3. Differentiate between: a) Work and energy b) Work and power c) Kinetic energy and potential energy d) Work from burning fuel and work from moving objects 4. Answer the following questions. (a) Define work. Write its SI unit. Is work scalar or vector quantity? (b) State a situation in which force is applied to a body, but no work is done. 68 New Creative Science and Environment Book - 8

(c) A man is standing for the whole day feels tired but no work is said to be done by him. Why? (d) On which factors does the kinetic energy depend? (e) What form of energy is contained in the following? • food • a stone at height • fuel • magnet • moving vehicle • sun (f) A bulb is marked 60 watt. What do you mean by it? (g) Name the different forms of energy. (h) Write the principle of conservation of energy. Numerical Problems 1. Find the amount of work done when a body moves at a distance of 2 m by the application of 200 N force. (Ans: 4000 Joule) 2. Find the kinetic energy of the body of mass 40 kg moving with the velocity of 20 m/s. (Ans: 8000 Joule) 3. How much work is done by a man if he moves a distance of 15 m by applying the force of 20N? Find his power if he does the work in 2 seconds. (Ans: 300 J, 150 W) 4. A porter can carry 40 bricks at a time. If the weight of each brick is 10 N, find his power if he carries these bricks up to 70 m in 50 seconds. (Ans: 560 W) 5. In a room a 60 watt bulb is used for ten hours. How much energy is consumed by it? (Ans: 2.16 × 106 J) 6. A person of 60 kg climbs the stair of total height of 20 m in 2 minutes, calculate his power. Take g = 10 ms–2. (Ans: 100 w) lossar – capacity of doing work distan e vered in a ed dire n Energy – to convert into small pieces D isplacement – to pull something F ission – to combine to make big D rag – to change from one form to another F usion – to save rans r a n nserva n Work, energy and power 69

U 6 HEAT L earning O utc om es At the end of this unit, students will be able to: ~ define heat and temperature, and show their relation. ~ state the method on how to establish relation of different temperature units (Celsius, Fahrenheit). ~ show the relation of different temperature units and do a simple conversion. ~ explain the structure and working of a simple thermometer and a clinical thermometer. Main points to be focused ~ Determination of the lower fixed point ~ Heat ~ Calibration of thermometer ~ Temperature ~ The temperature scales ~ Thermometer ~ Relation between the temperature scales ~ Construction of thermometer ~ Types of thermometers ~ Determination of the upper fixed point Introduction If we touch a hot cup of tea, energy enters our hand and we feel hot. Similarly, when we touch a piece of ice, energy transfers from our hand to the ice. So, we feel cool. Likewise, if we place a beaker of water over a heater, the water first becomes warm and finally begins to boil. This shows energy is transferred from the heater to the water and causes the water to become warm. This energy is called heat. Thus, heat is a form of energy, which gives us the sensation of warmth. It flows from a hotter body to a colder body. The sun is the main source of heat. Heat is produced due to the vibration of the molecules of a body. Hence, heat is the sum of the total kinetic energy of the molecules. The more the kinetic energy is, the more will be the heat contained in a body. The SI unit of heat is joule as it is also a form of energy. It is measured by a calorimeter in calorie. Calorie is the convenient unit of heat. 70 New Creative Science and Environment Book - 8

Memory Note ; 1 calorie heat = 4.2 joules ; One calorie heat is the amount of heat required to raise the temperature of 1gm water through 1°C. (Especially from 14.5°C to 15.5°C) Temperature Temperature is the degree or measure of the hotness and coldness of a body. When a body is heated, the molecules of the body begin to vibrate faster and thus the average kinetic energy of each molecule increases. Thus, temperature is the average kinetic energy of the molecules of the body. It is measured by an instrument known as a thermometer. The temperature is measured in different units like degree centigrade or degree Celsius (°C), degree Fahrenheit (°F), degree reumer (°R), etc. But its SI unit is kelvin. Heat and temperature The more the heat on a body is, the more will be the average kinetic energy of the molecules, and hence we will have more temperature. Thus, we can say that the temperature is directly proportional to the amount of heat contained in a body. For example, if we pour a cup of water from a kettle containing warm water then both the water in the cup and in the kettle will have the same temperature. However, the amount of heat contained in the water of the kettle will be more. Since, it contains more water, the total kinetic energy of the water molecules is more in the kettle than in the cup. Differences between heat and temperature Heat Temperature 1. It is a form of energy, which gives 1. It is the degree or the measure of us the sensation of warmth. hotness and coldness of a body. 2. It is the sum of the total kinetic 2. It is the average kinetic energy of the energy of the molecules of a body. molecules of a body. 3. It is a cause. 3. It is an effect. 4. It is measured by a calorimeter. 4. It is measured by a thermometer. 5. Its SI unit is joule. 5. Its SI unit is kelvin. Heat 71

Memory Note ; The earliest thermometer was developed by Galileo. Questions i. Show that heat is a form of energy. ii. Differentiate between heat and temperature on the basis of the kinetic theory of molecules. Thermometer A thermometer is an instrument that measures the temperature of a body. It is based on a principle that, ‘a substance expands on heating and contracts on cooling. Heat increases volume of the substance. It is directly proportional to the rise in temperature.’ Different substances are used in a thermometer, which show the temperature. They are known as thermometric substances. Generally, liquids like alcohol and mercury are used as the thermometric substances. Mercury is used as the thermometric liquid because of the following properties. i) It is a good conductor of heat. ii) It expands and contracts uniformly. iii) It doesn’t wet the glass capillary. So, the reading is correct. iv) It has high density. So, a small bulb or a short glass tube is sufficient to contain enough mercury. v) Its temperature changes very fast. It has low specific heat capacity. vi) It can measure a wide range of temperature as its freezing point is -39°C and boiling point is 357°C. vii) It has silvery colour. Thus, it can be read easily inside the glass capillary. However, despite these advantages, mercury has some disadvantages too. As we have noticed its freezing point is –39°C, it is not suitable to measure the temperature in cold regions when the temperature is below –39°C. Another disadvantage is that the mercury is highly toxic. So, it should be handled with great care. 72 New Creative Science and Environment Book - 8

Questions i. Why is a mercury used as the thermometric liquid? ii. Why is a mercury thermometer not suitable to measure the temperature in very cold regions? Alcohol as the thermometric substance Another commonly used thermometric substance is alcohol. It is used in thermometers due to the following properties: i) Its freezing point is very less i.e. – 117°C. So, it is suitable to measure the temperature of very cold places. ii) It expands seven times more than mercury. So, the reading is more correct. But it has also many disadvantages like: i) It is a colourless liquid and is a bad conductor of heat. ii) It sticks on the wall of the capillary tube. iii) It doesn’t expand uniformly. iv) It has a very low boiling point i.e. 78°C. So, it is not suitable even to measure the boiling point of water. v) It has low density. Question i. We prefer to use alcohol thermometers in a very cold region. Why? Water as the thermometric liquid Water is not suitable to use as a thermometric liquid because of the following reasons: i) The pure water is a bad conductor of heat. ii) It sticks on the wall of the capillary. iii) It has low density. iv) Its expansion rate is not uniform. v) Its specific heat capacity is very high. So, the rise in its temperature is very low. Memory Note ; The more the specific heat capacity of a substance is, the less will be its rate of change in temperature. Water (sp. heat capacity 4200 J kg–1° C–1) gets heated or cooled slower than the mercury (sp. heat capacity 138 J Kg–1°C–1) Heat 73

Questions i. Why can water be used as the thermometric liquid? ctivit Take two beakers of the same size. Pour 50 gm water into one of the beakers and 100 gm water into another. Measure the temperature of both the beakers. Now by using the same size of the spirit lamp, heat these two beakers. Note their temperature in every two minutes. Whose temperature increases faster? Why? Construction of thermometer For the construction of a liquid thermometer, first a glass tube with a uniform fine bore is taken in which a bulb is attached to one end. Mercury is filled in the bulb and the tube. Another end of that tube is sealed. Now the upper fixed point and the lower fixed point of the thermometer are found. Determination of the upper fixed point The temperature at which the pure water boils at the normal atmospheric pressure (i.e. 760 mm of Hg) is called the upper fixed point. Its value is 100°C or 373 K or 212°F. To determine the upper fixed point, the bulb of the thermometer should be kept above the level of the boiling water, and the apparatuses are arranged as shown in the figure. Measuring upper fixed point 74 New Creative Science and Environment Book - 8

There should be an outlet for the steam. The bulb of the thermometer should be above the level of the water in the flask. The level of the mercury in the thermometer begins to rise and becomes stationary at a fixed point. This is the upper fixed point of the thermometer. Now, it is marked. Determination of the lower fixed point The temperature at which the pure ice melts at the normal atmospheric pressure (760 mm of Hg) is called the lower fixed point. Its value is 0°C or 273 K or 32°F. To determine the lower fixed point, the bulb of the thermometer should be kept in contact with the melting ice in a funnel as shown in the figure. Measuring lower fixed point There should not be air gaps between the bulb and the ice. This is because if there is a gap then the thermometer measures the temperature of the air not that of the melting ice. The level of mercury in the thermometer falls and becomes stationary at a fixed point. This is the lower fixed point of the thermometer, and it is marked. Calibration of thermometer After finding the upper fixed and the lower fixed points of the thermometer, the gap between these two points is divided into small and equal divisions. This process is called the calibration of the thermometer. The calibration is done according to the temperature scale used. The temperature scales The commonly used temperature scales are the Celsius, Fahrenheit and Kelvin scale. Heat 75

i) The Celsius scale This scale was made by Celsius (1701-1744). Its lower fixed point is 0°C and the upper fixed point is 100°C. The gap between these two fixed points is divided into 100 equal parts. Each division is equal to 1°C. The normal temperature of the human body is 37°C. ii) The Fahrenheit scale This scale was developed by Fahrenheit (1686-1736). Its lower fixed point is 32°F and upper fixed point is 212°F. The gap between them is divided into 180 equal parts. Each part equals to 1°F. The normal temperature of the human body is 98.6°F. iii) The Kelvin scale Its lower fixed point is 273 K and upper fixed point is 373 K, and the gap between them is divided into 100 equal parts. Each part equals to 1K. Celsius scale Fahrenheit scale Kelvin scale Relation between the temperature scales The upper fixed and the lower fixed points of the thermometers are the same though they are marked differently. Let C, F and K be the temperature readings on Celsius, Fahrenheit and Kelvin scale. Then the relation between these temperature scales is given as, C–0 = F – 32 = K – 273 100 180 100 76 New Creative Science and Environment Book - 8

Solved Numericals 1. Convert 104°F to Celsius scale: Solution: We have, C–0 ===F17–182003141×2881–0003021 Or, C– 0100 Or, 100 C ∴C = 40 104°F = 40°C 2. Calculate the temperature, which is common to both i.e. Celsius and the Fahrenheit scale. Solution: Let, x be the common temperature. Using, C–0 = F – 32 or, X100 = X1–8032 100 or, 180 x 180 = 100 x – 3200 or, 80 x = – 3200 or, x = –3200 ∴ x = – 40° ∴ – 40°C 80 = – 40°F 3. The temperature of warm water in a beaker is 60°C. Convert it into Kelvin scale. Solution: We have, C–0 = K – 273 100 100 or, C = K – 273 or, 60 = K – 273 or, K = 273 + 60 or, K = 333 ∴ 60°C = 333 K Types of thermometers Thermometers are classified according to their thermometric substances like i) liquid thermometer ii) gas thermometer, iii) radiation thermometer, iv) thermoelectric thermometer, v) resistance thermometer etc. Similarly, according to their purposes, thermometers are classified into: i) Laboratory thermometer ii) Clinical thermometer iii) Maximum and minimum thermometer Heat 77

i) Laboratory thermometer It is a thermometer used to measure the temperature of the air, water and other various substances in the laboratory. It is mainly round and long. The mercury rises and falls automatically in this thermometer after being hot and cold respectively. The wall of the bulb is made thin and the capillary is made narrow for the quick response to the change in temperature. Laboratory thermometer The stem is graduated from –10°C to 110°C. For measuring the temperature, the bulb is kept in contact with the substance whose temperature is going to measure. Now the change in mercury level is noticed. There is a vacuum above the mercury surface in the capillary tube, which makes it easier for the mercury to rise. Question i. Why is the bulb of a laboratory thermometer made thin? ii) Clinical thermometer The thermometer which is used to measure the body temperature is called a clinical thermometer or a doctor’s thermometer. It is comparatively shorter than the laboratory thermometer and is prismatic in shape. So, the prismatic shape of a clinical thermometer gives a magnified and clear view of the mercury level inside the capillary. There is a constriction above the bulb of the mercury in the capillary. This helps to increase the mercury level when heated but doesn’t allow the falling of the mercury level even when the temperature falls. So, it helps to take the correct temperature of the body for a long time after taking out from the human body. ( kink) Normal body temperature Clinical thermometer The stem is graduated from 35°C to 42°C or, 94°F to 108°F. There is a mark at 37°C or 98.6°F. It is the normal human body temperature. For measuring temperature, the bulb is kept in contact with the human body by keeping it in the armpit or below the tongue, and it should be taken out after about 2 minutes to read the body temperature. It should be jerked before using it again to put the mercury level in the initial position because the constriction does not allow it to fall automatically. 78 New Creative Science and Environment Book - 8

Question i. Why must a clinical thermometer be jerked before using it again? Differences between clinical and laboratory thermometers Clinical thermometer Lab thermometer 1. It is used to measure the body 1. It is used to measure the temperature of temperature. various substances in the lab. 2. It has constriction in its stem. 2. It doesn’t have construction in its stem. 3. The stem is graduated from 3. The stem is graduated from –10°C to 35°C to 42°C. 110°C. 4. It is prismatic in shape. 4. It is round and cylindrical in shape. iii) Maximum and minimum thermometer It is a thermometer used to measure the maximum and minimum temperatures in a certain place within 24 hrs. It consists of a U-shaped tube partly filled with mercury and partly filled with alcohol. One arm of this thermometer is connected with a large bulb filled with mercury, and the other arm is connected with a small bulb, which is also filled with alcohol. One arm shows the maximum temperature and the other arm shows the minimum temperature. Each arm has an index to show the temperature. When the temperature increases, the alcohol in the large bulb expands. Due to this, the mercury level along with the pointer moves upward in the other arm to show the maximum temperature. When the temperature falls, the alcohol contracts and the mercury level along with pointer moves upward in the same arm and shows the minimum temperature. Once the pointers set to the maximum and minimum temperatures, they do not fall themselves. To reset the thermometer, both the indexes have to be pulled down with the help of a magnet as they are adjusted just above the mercury surfaces. Answer writing skills 1. How can the sensitivity of a thermometer be increased?  The sensitivity of a thermometer can be increased if the bulb of the thermometers is constructed with the largest surface area, and making the capillary tube very thin. Heat 79

2. Why is mercury used as a thermometric substance?  Mercury is used as a thermometric substance because: i) It is a good conductor of heat. ii) It doesn’t make the wall of the glass tube wet. iii) It has silvery colour. iv) It has high density, etc. 3. A clinical thermometer should be jerked before using it again. Why?  There is a constriction in a clinical thermometer. It allows the increase in the mercury level when gets heated and breaks the mercury level. It cannot fall automatically even when the temperature is lowered. So, to reset the thermometer to use again, it should be jerked. 4. The temperature of boiling water cannot be measured by using a alcohol thermometer. Why?  Alcohol has the boiling point of 78°C but water boils at 100°C. Alcohol itself evaporates before measuring the temperature of the boiling water. So, alcohol cannot be used to measure the temperature of the boiling water. 5. An alcohol thermometer is preferred for very cold region. Why?  Alcohol has the freezing point of –117°C. So, it can measure the temperature of very cold regions where the temperature falls below – 40°C. 6. What is the importance of thermometer?  The thermometers are important because: › They are used to measure the body temperature. › They are used to measure the temperature of water and room temperature. › They are also used to note the maximum and minimum temperature of a certain place. 7. What are the roles of an index in the maximum-minimum thermometer?  The values indicated by the lower ends of the indexes are the maximum and minimum temperature. So, these indexes help to show the maximum and minimum temperatures. SUM M ARY › Heat is a form of energy, which gives us the sensation of warmth. › The degree of hotness and coldness of a body is called temperature. › The instrument which is used to measure the temperature of a certain place is called a thermometer. 80 New Creative Science and Environment Book - 8

› Mercury is the commonly used thermometric substance. › Thermometers are constructed on the principle that, ‘a substance expands on heating and contracts on cooling.’ › The temperature at which pure ice melts at normal atmospheric pressure (760 mm of Hg) is called the lower fixed point of the thermometer. › The temperature at which pure water boils at normal atmospheric pressure is called the upper fixed point. › The main temperature scales are: centigrade scale, Fahrenheit scale and the kelvin scale. › The relation between the temperature scales is C–0 F – 32 = K – 273 100 = 180 100 › A clinical thermometer has constriction just above the bulb. › A laboratory thermometer is long and round. › A maximum and minimum thermometer is used to measure the maximum and minimum temperature of a certain place within 24 hrs. The normal human body temperature is 98.6°F or 37°C. Exercise 1. Fill in the blanks. (a) The upper fixed point of pure water is ___________. (b) The normal temperature of a human body is _________. (c) Heat is a form of energy, which gives us the sensation of _________. (d) ________ thermometer is used to measure the temperature of a human body. (e) _______ thermometer is used to measure the temperature in very cold regions. 2. Write True for correct and False for the wrong statements. (a) A hotter body transfers heat to a colder body. (b) 1 kilo calorie is equal to 4.2 joules. (c) The freezing point of water is 100°C. (d) Heat is a form of energy. (e) Temperature is measured in calorie. Heat 81

3. Answer the following questions. (a) Define heat. Write its SI unit. What do you mean by 1 calorie heat? (b) Define temperature. What is its SI unit? (c) Differentiate between heat and temperature. (d) Differentiate between a clinical and a laboratory thermometer. (e) On which principle is a thermometer based? (f) Describe the clinical thermometer with a figure. (g) Which thermometer do you prefer to measure the temperature of boiling water between a clinical or a laboratory thermometer and why? (h) Write the relationship between the temperature scales. (i) What are the advantages of using mercury in a thermometer? (j) What are the advantages and disadvantages of using alcohol as the thermometric liquid? 4. Give reasons. (a) A alcohol thermometer is used in very cold regions. (b) Constriction is made in a clinical thermometer. (c) A clinical thermometer should be jerked before using it again. (d) Steel pointers are used in a maximum and minimum thermometer. Numerical problems 1. Convert 100°C to Kelvin scale 2. Convert 37°C to Fahrenheit scale 3. Convert 107°F to Celsius scale 4. Convert 273 K into Fahrenheit scale lossar – in the shape of a prism – temperature at which a pure substance freez es ris a – temperature at which a pure substance melts F reez ing point – the amount of heat req uired for 1 kg mass to change its temperature by 1° C . e n p int Spe i heat apa it 82 New Creative Science and Environment Book - 8

U 7 LIGHT L earning O utc om es At the end of this unit, students will be able to: ~ define a mirror and its types (plain mirror and spherical mirror) and demonstrate the reflection from a spherical mirror. ~ define and demonstrate real and virtual images. ~ draw and demonstrate the ray diagrams keeping the object at a different distances (at infinite and behind the centre of curvature) from the spherical mirror. ~ explain the uses of spherical mirror. Main points to be focused ~ Use of concave mirror ~ Ray and Beam of light ~ Use of convex mirror ~ Mirror ~ Refraction of light ~ Concave mirror ~ Effects of refraction ~ Convex mirror ~ Construction of ray diagram Introduction Light is a form of energy which gives us the sensation of vision. Due to this, we are able to see around and enjoy the beauty of the nature. During the day, we are able to see due to the sunlight and at night, we need the other sources of light to see around such as glowing bulbs, tube lights, burning candles, etc. The things which produce light are called its sources. The sun is the main source of light on the earth. The sources of light may be natural and artificial. The sun, moon, junkiri, stars, etc. are the natural sources of light. Similarly, burning candles, lamps, tube lights, etc. are the artificial sources of light. The things which emit their own light are called the luminous bodies, e.g. the sun, stars, junkiri, etc. Whereas the bodies which do not have their own light are called the non- luminous bodies, e.g. brick, wood, moon, planets, etc. Light 83

Ray and Beam of light The smallest part of light is known as the ray. It is also called the single narrow path of the light. Since, the light travels in a straight line in one medium, a ray is represented by a straight line with an arrowhead. The arrowhead shows the direction of propagation of light. A ray of light Beam The collection of rays is called a beam. There are three types of beams of light. They are: i) Parallel beams ii) Convergent beams iii) Divergent beams (i) Parallel beam If the rays of light are parallel to each other, the beam is known as the parallel beam. For example, the beam of sunlight. (ii) Convergent beam If the rays meet at a single point, it is called the convergent beam. For example, the rays through the convex lens. (iii) Divergent beam If the rays scatter from a single point, it is called the divergent beam. For example, the rays reflected through the candle, bulb etc. Question i. Differentiate between convergent and divergent beams of light. Memory Note ; Light is the fastest traveller. Its velocity in air or vacuum is 3 × 108 m/s. Yet, it takes about 8 minutes to reach the earth from the sun. The distance between the heavenly bodies in the universe is so large that even the light takes hundreds of years to cover that distance. 84 New Creative Science and Environment Book - 8

Mirror It is a reflecting surface whose one surface is polished. It forms the image of an object after the reflection of light through it. There are two types of mirrors. They are: i) Plane mirror ii) Curved mirror (i) Plane mirror It is a mirror whose reflecting surface is plane or flat. The image formed by a plane mirror is erect and virtual. It means that the image cannot be taken in the screen. It forms the image of the same size of that object but the image is laterally inverted. It means the image is sideways inverse and the left side appears right and vice-versa. Stand in front of a plane mirror and raise your right hand. You will see as if you have raised your left hand. The plane mirror is used as a looking glass in the house, beauty parlours and saloons. It is also used to make a periscope and used for various purposes in the physics laboratory. (ii) Curved mirror The mirror whose reflecting surface is curved is called curved mirror. They are also known as spherical mirrors. They are the part of the surface of a sphere and also obey the laws of reflection. There are two types of curved mirrors. They are (a) concave mirror and (b) convex mirror. (a) Concave mirror The spherical mirror in which its inner surface acts as the reflecting surface and the outer surface is made opaque by coating it with a layer of silver is called a concave mirror. The concave mirror is also known as a converging mirror. This is because a concave mirror converges the rays of the light falling on it to a single point, e.g. reflecting the surface of a torch, headlights of vehicles, etc. Light 85

(b) Convex mirror It is a spherical mirror whose inner surface is coated and the outer surface acts as the reflecting surface. It is also known as the diverging mirror because it scatters the rays of the light falling on it, e.g. mirrors used in the vehicles as side view mirror, etc. Some terms related to the spherical mirrors (i) Pole of the mirror [‘O’ or ‘P’] The geometric centre of a spherical mirror is called its pole. In the figure, ‘P’ is the pole of the mirror. (ii) Centre of curvature The centre of the sphere of which the curved mirror forms is called the centre of curvature. In the figure, ‘C” is the centre of curvature. As seen clear from the figure that the centre of curvature of concave mirror is in front of the reflecting surface while it lies behind the reflecting surface in a convex mirror. (iii) Principal Axis The line passing through the pole and the centre of curvature of the spherical mirror is called its principal axis. In figure, PC is the principal axis. (iv) Principal focus It is the point on the principal axis to which the parallel rays of light striking the concave mirror converge at a point, and for a convex mirror the parallel rays of light striking the surface appear to meet. It is denoted by F. In a concave mirror, the focus lies in front of the reflecting surface and in case of a convex mirror, it lies behind the reflecting surface. Thus, a concave mirror has a real principal focus while a convex mirror has a virtual one. (v) Focal length The distance between focus and the pole of the mirror is called its focal length. It is denoted by f. (vi) Radius of curvature The distance between the pole of the mirror and the centre of curvature is called the radius of curvature. In the figure, PC is the radius of curvature and it is represented by r. 86 New Creative Science and Environment Book - 8

Memory Note ; The radius of curvature of a mirror is twice its focal length. i.e. r = 2f Question i. Why is a concave mirror called a converging mirror and a convex mirror a diverging mirror? ctivit Take a stainless steel spoon. Bring the out part of the spoon near your face and look into it. Do you see your image in it? Is this image different from what you see in a plane mirror? Is this image erect? Is the size of the image the same, smaller or larger? Now, look at your image using the inner side of the spoon. This time you may find that your image is erect and larger in size. If you increase the distance of the spoon from your face you may see your image inverted. These results can be proved by constructing ray diagrams. Construction of ray diagram A ray diagram is a simplified diagram to show the position and nature of an image formed by the mirror. Rules for the construction of ray diagrams by a concave mirror The following information should be remembered while constructing ray diagrams: (a) When a ray of light from an object comes parallel to the principal axis then the ray passes through the focus after getting reflected. (b) A ray of light from an object passing through the focus is reflected parallel to the principal axis. Light 87

(c) The light rays emerging from the centre of curvature makes 90° with surface of the mirror and reflect back along the same path. Steps for construction of a ray diagram i) Draw a spherical mirror by using a compass and a pencil. The point of the needle is the centre of curvature. Now, shade the outer region to make the concave mirror. ii) Locate the centre of the lens and draw a straight line joining the centre of curvature and this pole. iii) Place the object in front of the mirror on the principal axis. iv) Draw a parallel line to the principal axis from the object and reflect it by passing it through the focus. v) Draw another line through the focus and reflect it back parallel to the principal axis. vi) An image is formed at the point, at which the two reflected rays meet. Let’s try this by drawing a ray diagram by placing the object at various places before the concave lens. (1) Object at infinity Nature and position of image (a) The image is formed at focus. (b) The image is real, inverted and highly diminished. (2) Object beyond C Nature and position of image (a) The image is formed between ‘C’ and ‘F’. (b) The image is real, inverted and diminished. 88 New Creative Science and Environment Book - 8

(3) Object at ‘C’ Nature and position of a image O bject (a) The image is formed at ‘C’. (b) The image is real, inverted and of the same I mage size of the object. (4) Object between ‘C’ and ‘F’ O bject Nature and Position of image I mage (a) The image is formed beyond ‘C’. (b) The image is real, inverted and magnified. (5) Object at focus (F) O bject Nature and position of a image (a) The image is formed at infinity. I mage (b) The image is real, inverted and highly magnified. (6) Object between F and P I mage Nature and position of a O bject image (a) The image is formed behind a mirror. (b) The image is virtual, erect and magnified. This is the only case in which the concave mirror forms a virtual and erect image. All the above observations of a concave mirror are summarized in the following table. S.No. Position of object Position of image Nature of image 1. At infinity At focus Real, inverted, smaller. 2. Between infinity and C Between C and F Real, inverted, smaller. 3. At C At C Real, inverted, equal. Light 89

4. Between C and F Between C and infinity Real, inverted, enlarged. 5. At F At infinity Real, inverted, enlarged. 6. Between F and pole Behind the mirror Virtual, erect, enlarged. Uses of concave mirror (i) They are used for converging the sunlight in solar cookers. (ii) They are also needed in torches, searchlight and in the headlights of cars and other vehicles. (iii) They are used in the construction of an astronomical telescope. (iv) When a person keeps his face between the focus and pole of a concave mirror, it forms a virtual, erect and magnified image. This property of the mirror is used to make shaving glasses or mirrors. Questions i. Why is a concave mirror used in the headlights of vehicles? ii. Why is a concave mirror used as a shaving glass? Rules for constructing the ray diagram by a convex mirror (i) A parallel ray to the principal axis appears to pass through the focus. (ii) A ray of light coming along the line through the centre of curvature retraces its path. (iii) A ray of light striking the pole of the mirror at a certain angle reflects at the same angle. Image formation by a convex mirror In the case of a convex mirror, the image is always virtual, erect and diminished. The image is formed behind the mirror. 90 New Creative Science and Environment Book - 8

O bject Uses of convex mirror (i) They are used to have a back view mirrors in automobiles. (ii) A combination of concave and the convex mirrors is often used in fairs, fun parks and children centres for showing strange looking images of the person who stands in front of them. (iii) Convex mirrors are also used in street lights. Differences between convex mirror and concave mirrors S.N. Concave mirror S.N. Convex mirror i. i. Its outer surface is the reflecting ii. Its inner surface is the reflecting ii. surface. surface. It always forms a virtual iii. iii. image. It mainly forms a real image except when the object is kept between P It always forms a diminished image. and F. It is used in vehicles as rear back It may form a magnified or view mirrors. diminished image. It is used as a shaving mirror, to construct astronomical telescopes, etc. Differences between a real image and a virtual image S.N. Real image S.N. Virtual image i. It is the image formed by the actual i. It is the image formed by the apparent intersection of the reflected rays. intersection of the reflected rays. ii. It is always inverted. ii. It is always erected. iii. It can be obtained on the screen. iii. It cannot be obtained on the screen. iv. It is formed in front of the mirror. iv. It is formed behind the mirror. Light 91

ctivit Visit a nearby hospital. You can also visit the clinic of an ENT specialist, or a dentist. Request the doctor to show you the mirrors used for examining the ear, nose, throat and teeth. Can you recognize the kind of mirror used in these instruments? Refraction of light A pond appears shallower than it actually is. Similarly, when a pencil is partially dipped into water, it appears bent. It is all due to the refraction of light. Refraction is the phenomenon of the bending of the light at the junction of two media when the light travels from one medium to another. As we know, light always travels in a straight line. But this is true only when the light is travelling in one medium. But when the light passes from one medium to another, it bends at the junction. It is due to the difference in the densities of these two mediums. The speed of light in a denser medium is less than the speed in a rarer medium. Due to this, light bends. Memory Note ; Stars twinkle due to the refraction of light. Question i. Why does light bend while travelling from one medium to another? Some terms related to the refraction of light When a ray of light is passed through the glass slab, it gets refracted as shown in the figure. Here, PQRS is the glass slab. (i) Incident ray The ray which comes to incident on the glass slab is called the incident ray. In the figure, AO is the incident ray. (ii) Refracted ray R arer medium The ray which bends is called the refracted ray. In the figure, OB is the refracted ray. S (iii) Normal The line drawn perpendicular to the glass slab is called normal. In figure, MN is the normal. 92 New Creative Science and Environment Book - 8

(iv) Emergent ray The ray which turns back to air after refraction through the glass slab is called the emergent ray. In the figure, BC is the emergent ray. (v) Angle of incidence (i) The angle between the incident ray and the normal is called the angle of incidence. In the figure, ∠AOM is the angle of incidence. (vi) Angle of refraction The angle between the refracted ray and normal is called the angle of refraction. In the figure, ∠BON is the angle of refraction. Memory Note ; The velocity of light in air, water and glass is 3 × 108 m/s, 2.2 × 108 m/s and 2 × 108 m/s respectively. Law of refraction of light (i) The incident ray, refracted ray and the normal lie on the same plane at the same point of incidence. (ii) The ratio of the sine of angle of incidence to the sine of angle of refraction for the given two mediums is always a constant i.e. sin i = µ [ Where µ is a constant known as the refractive index of these two media sin r (iii) • When the light passes from a denser medium to a rarer medium, it bends away from the normal. • When the light passes from a rarer medium to a denser medium, it bends towards the normal. • When the light passes along the normal, it passes through without bending. A ir rarer Air rarer Air rarer Light 93

Effects of refraction A pencil appears bent upward when it is partly immersed in water. ctivit Take a beaker nearly half filled with water. Dip a pencil half inside the water and observe it from the top and side and draw the diagram to show how it looks. When a pencil is dipped into water, the rays of the light coming from the pencil inside the water bend away from the normal. As light comes out of the water level the light has travelled from a denser (water) to a rarer (air) medium. When the refracted rays are produced backwards, they meet at ‘C’. Thus, C is the image of the tip of the pencil. Similarly, it is true for all other points of the pencil. So, the observer will see AC as the apparent image of AB. Hence, the pencil appears to be bent. A pond appears shallower than it actually is. › When the light from the objects at the bottom of the pond comes out to the air, it bends away from the normal, as it is passing through a denser medium to a rarer medium. So, when the refracted rays are traced backwards, they meet at the lesser depth. So, the pond appears to be shallower than it actually is. 94 New Creative Science and Environment Book - 8

Memory Note ; Galileo Galilei was born on 15 February 1564 in Pisa, Italy. He developed a series of telescopes where optical performance was much better than that of other telescopes available during those days. Answer writing Skills 1. Why is a convex mirror called a diverging mirror and the concave mirror converging mirror?  The concave mirror converges parallel beam of light to a single point. So, it is called the converging mirror. The convex mirror diverges the light falling on it to different directions. So, it is also known as a diverging mirror. 2. Why is a concave mirror used in the headlights of vehicles?  In the headlight, the bulb is kept at about the focus of the concave mirror. So, the light falling on it gets reflected parallel to the principal axis and thus, we can see it at a larger distance. 3. A convex mirror is used as the back rare view in vehicle. Why?  Convex mirror forms the virtual, erect and diminished image of the objects. So, it covers a wide range of view and the drivers can easily see the traffic behind his vehicle. So, the convex mirror is used as the back rare view in the vehicle. 4. Which mirror is used as a shaving mirror and why?  A concave mirror is used as a shaving mirror as it forms an erect and enlarged image if the object is placed between its pole and focus. 5. We use a plane mirror to see our face in our daily life.  We use a plane mirror to see our face because it forms the virtual and the erect image of the same size of the object. Light 95

6. A pond appears shallower than it actually is.  When the light from the object at the bottom of the pond comes out of the water, it bends away from the normal at the surface of water. When the refracted rays are traced backwards they meet at a point at the lesser depth, and thus, the pond appears shallower than it actually is. SUM M ARY › Light is a form of energy which gives us the sensation of vision. › The sun, moon, stars are the natural sources and a burning candle, an electric lamp, tube lights are the artificial sources of light. › A ray is the smallest part of the light which is represented by a straight line with an arrowhead. › A beam is the collection of rays. It may be convergent, divergent or parallel. › The process of the turning back of light after striking any obstacle on its way is called the reflection of light. › The reflection of light may be a regular reflection and an irregular reflection. › The mirror whose reflecting surface is plane is called a plane mirror and the one which has the curved reflecting surface is called a curved mirror. › The curved mirror may be concave or convex. › The spherical mirror whose inner surface is coated and outer surface acts as a reflecting surface is called a convex mirror and the mirror whose inner surface acts as a reflecting surface is called a concave mirror. › Focus is the point on the principal axis to which the parallel rays of light striking the concave mirror converge and from which the parallel rays of light striking the convex mirror appear to diverge. › A convex mirror is used as rear back view mirrors in automobiles and a concave mirror is used in torches, searchlight and headlights of vehicles. › The process of the bending of light at the junction of two mediums when the light travels from one medium to another is the refraction of light. Exercise 1. Fill in the blanks. (a) The angle of incidence is equal to the angle of ____________. 96 New Creative Science and Environment Book - 8