202110206-APEX-STUDENT-WORKBOOK-PHYSICAL_SCIENCE-G08-PART2

(A) CNG (B) LPG (C) PGL (D) NCG AS6-Appreciation and aesthetic sense, Values 24. Choose the correct answer. (i) is an excellent fire extinguisher. (B) Hydrogen dioxide (A) Oxygen (C)Carbon dioxide (D) Nitrogen AS7-Application to daily life, concern to bio diversity 25. Choose the correct answer. (i) is a source of both light and usable heat. (A) Candle (B) Tube light (C) Bulb (D) CFL CHAPTER 8. COMBUSTION, FUELS AND FLAME 49

9. ELECTRICAL CONDUCTIVITY OF LIQUIDS SESSION 1 INTRODUCTION AND ELECTRICAL CONDUCTIVITY OF LIQUIDS 1.1 Mind Map 1.2 Terminology i. Good conductors – A good conductor of electricity allows an easy flow of electric current through them. E.g., – Copper, Silver etc. ii. Poor conductors – Poor conductors do not allow current to pass through them. E.g., – Rubber, Plastic etc. iii. Circuit – In electronics, a circuit is a path between two or more points along which an electrical current can be carried. iv. Tester – Tester is a simple piece of electronic test equipment used to determine the presence or absence of an electric voltage in a piece of equipment under test. SESSION 1. INTRODUCTION AND ELECTRICAL CONDUCTIVITY OF LIQ... 50

v. Dry cell – A voltage–generating cell having an electrolyte in the form of moist paste. 1.3 Key Concepts i. Conductors of electricity are those materials that allow electricity to pass through them. ii. Insulators (poor conductors) of electricity are those materials that do not allow elec- tricity to pass through them. iii. Many materials around us are conductors and some are poor conductors. iv. Some liquids are good conductors of electricity and some are poor conductors. v. Pure water does not allow the current to pass through. 1.4 Conceptual Understanding Q1. Give examples for good solid conductors and liquid conductors. [Refer to TB page 132 Q1] A. Good solid conductors: Copper, silver, iron, aluminium, graphite etc. Good liquid conductors: Drinking water, lemon juice, mercury, acid solution etc. Q2. Give examples for bad solid conductors and liquid conductors. [Refer to TB page 132 Q2] A. Poor solid conductors: Plastic, rubber, paper etc. Poor liquid conductors: Distilled water, honey, oil, etc. Q3. What do you add to distilled water for making it to conduct electricity? [Refer to TB page 132 Q3] A. We can add salt to distilled water to make it conduct electricity. Q4. In case of a fire, before the firemen use the water, they shut the main electrical supply for the area. Explain why they do this. [Refer to TB page 133 Q7] A. Water conducts electricity. If the electrical supply for the area is not shut off and water is poured over electrical appliances, then electricity may pass through the water and they may have electric shock. So the firemen shut off the main electrical supply for the area before they use water. SESSION 1. INTRODUCTION AND ELECTRICAL CONDUCTIVITY OF LIQ... 51

1.5 Asking Questions and Making Hypothesis Q1. Kavya observed that a discharged dry cell which was kept in sunlight by her father for few hours got the ability to glow LED. She got many doubts and questions to raise. Can you guess those questions or doubts? [Refer to TB page 133 Q9] A. i. How could the bulb glow? ii. Which energy helped the bulb to glow? iii. How could the solar energy affect the cell? iv. How was electrical energy obtained? v. Can this be a repeated action? 1.6 Experimentation and Field Investigation Q1. Make a battery from four lemons and test it with an LED in the circuit. [Refer to TB page 133 Q1, Try These] A. MATERIALS REQUIRED: 4 lemons, 4 pieces of copper wire, 4 paper clips, 4 zinc plated nails, 4 wires and LED. i. Place a nail and a piece of copper into each lemon. ii. The copper acts as a positive terminal and nail as a negative terminal. iii. Join the 4 lemons using wire and clips such that nail of the first lemon is connected to copper of the second lemon and so on. iv. Add wires and clips to the first piece of copper and last piece of nail. v. Join one end of the leg of LED to wire of the first lemon and second leg of LED to wire of the last lemon. vi. You will see the dim glow of the light. SESSION 1. INTRODUCTION AND ELECTRICAL CONDUCTIVITY OF LIQ... 52

Q2. Refer to the Activity 3 in the chapter. Start with distilled water. The LED would not glow. Add two drops of some acid to distilled water and check the glow of LED. Add two more drops and check the intensity of the glow. Repeat the activity 5 to 6 times by adding two drops of the same acid each time. Do you see any difference in the intensity of glow with increasing acid content of water? What can we infer from the above observations? Repeat the entire activity by taking a solution of baking soda and adding drop of it to distilled water instead of acid. Write differences and similarities. [Refer to TB page 133 Q2, Try These] A. i. Observation: The LED does not glow with the distilled water because pure water (distilled) is a poor conductor of electricity. When we add 2 drops of acid to distilled water it starts glowing. As we are adding 2 drops of acid every time the intensity of LED increases. i. Inference: Distilled water is poor conductor of electricity. Addition of acid to distilled water makes it a conductor of electricity. As the acid content increases in the water, intensity of the glowing bulb increases. ii. Observation: With the addition of solution of baking soda LED starts glowing. Adding drops of solution increases the intensity of LED. ii. Inference: Solution of base is a good conductor of electricity. Similarities: Both are conductors of electricity. Dissimilarities: With the acid it glows more brightly. With the baking soda solution the glow is less. The reason is baking soda is a weak base. So the dissociation is less when compared with acid. So less ions are available for passage of electricity. So the intensity of glow of LED is less when compared with acid. SESSION 1. INTRODUCTION AND ELECTRICAL CONDUCTIVITY OF LIQ... 53

1.7 Communication Through Drawing and Model Making Q1. In many of the activities in this chapter, we have used a tester made up of LED. Can we avoid LED and use something else as a tester? Magnetic compass needle could be an alternative tester. We know that when we take a current carrying wire near magnetic compass needle, it shows deflection. Use this property to make a tester of magnetic compass needle. You may refer to the following figure.[Refer to TB page 133 Q3, Try These] A. Magnetic compass needle could be an alternative tester. We know that when we take a current carrying wire near magnetic compass it deflects. This property is used to make a tester using a magnetic compass needle. The magnetic needle deflects because whenever current pass through a wire it acts as a magnet and it is called magnetic effect of current. 1.8 Application to Daily Life, Concern to Bio Diversity Q1. Collect information and make a list of good conductors and bad conductors. How do you use this information in your daily life works? [Refer to TB page 133 Q12] A. Metals are good conductors and they are used in electrical wires, vessels, boilers, etc. Wood, rubber, plastic, etc., are poor conductors; they are used in switch boards, switches, insulation of wires, etc. or Good conductors are the ones that allow electricity to pass through them easily. A list of good conductors are silver, copper, gold, aluminium, magnesium, tungsten, nickel, iron, etc. SESSION 1. INTRODUCTION AND ELECTRICAL CONDUCTIVITY OF LIQ... 54

Bad conductors are the ones that do not allow the electricity to flow through them. A list of bad conductors are plastic, wood, wool, silk, paper etc. In daily life, we use copper for conducting electricity. All our electrical appliances such as fans, ovens, iron box etc need good conductors of electricity to work. At the same time, the materials such as copper which conduct electricity should not be in human contact which is enabled by insulating the same with bad conductors of electricity like plastic, wood etc. SESSION 1. INTRODUCTION AND ELECTRICAL CONDUCTIVITY OF LIQ... 55

SESSION 2 CHEMICAL EFFECTS OF ELECTRIC CURRENT AND ITS USES 2.1 Mind Map 1. Electrolytic cell SESSION 2. CHEMICAL EFFECTS OF ELECTRIC CURRENT AND ITS USES 56

2. Electroplating 2.2 Terminology i. Distilled Water – Distilled water is water that has many of its impurities removed through distillation. It is used in science laboratories for various purpose. ii. LED – A Light Emitting Diode is a semiconductor device that emits visible light when an electric current passes through it. iii. Positive Terminal – In a source of electrical energy, such as a battery, cell, or generator, the terminal to which electrons flow. 2.3 Key Concepts i. Most of the liquids which conduct electricity are solutions of acids, bases and salts. ii. The passage of current through a liquid causes chemical reactions. iii. Electrolyte is a solution of substances through which electric current can pass. iv. Electrolysis – Electrolysis is the process of breaking up of an electrolyte chemically by passing electricity through it. v. Electroplating – Electroplating is the coating of a thin layer of metal on another metal by the process of electrolysis. vi. Electrodes – A conductor through which electricity enters or leaves an object, sub- stance, or region. SESSION 2. CHEMICAL EFFECTS OF ELECTRIC CURRENT AND ITS USES 57

2.4 Conceptual Understanding Q1. What is an electrolyte? [Refer to TB page 132 Q4] A. An electrolyte is a compound which conducts electricity in fused state(molten) or in aqueous solution. Q2. Which energy is cause for glowing of bulb in an electrolytic cell? [Refer to TB page 132 Q5] A. Chemical energy Q3. Write the uses of electroplating. [Refer to TB page 132 Q6] A. i. Chromium plating is done on different parts of vehicles to give them lustre. ii. A fine layer of gold is deposited on cheap metals. iii. Iron used in construction is coated with a layer of zinc to protect it from corrosion and rusting. iv. Electroplating is also used in repairing worn out parts of machinery. Q4. We get some items made from iron wire in which iron wire is coated with plastic. Is plastic coated by a process of electroplating? Why plastic can’t be coated on a metal by the process of electroplating? [Refer to TB page 133 Q8] A. Plastic coated on iron wire is not the process of electroplating. Plastic is a poor conduc- tor of electricity and is a polymer and a covalent substance and cannot be decomposed into ions. Hence plastic cannot be coated on metal by the process of electroplating. 2.5 Experimentation and Field Investigation Q1. Explain the process of coating copper on an iron key? Draw the circuit diagram. [Refer to TB page 133 Q10] A. AIM: Coating an iron key with copper by electroplating method. MATERIALS REQUIRED: Copper plate, crystals of copper sulphate, an iron key, glass beaker, water, sulphuric acid, battery cell, and connecting wires. PROCEDURE: i. Dissolve copper sulphate crystals in water to prepare a concentrated solution and pour it in a beaker. SESSION 2. CHEMICAL EFFECTS OF ELECTRIC CURRENT AND ITS USES 58

ii. Add a few drops of dilute sulphuric acid to it. iii. Tie one end of copper wire to an iron key. Connect the other end to negative termi- nal of a battery. iv. Suspend the key and copper plate into the copper sulphate solution through posi- tive terminal of a battery through a switch. v. Care should be taken that the key and the copper plate should not touch each other. vi. Put on the switch for about 10–15 minutes. Copper gets deposited on the iron key. 2.6 Appreciation and Aesthetic Sense, Values Q1. How do you appreciate the efforts of Luigi Galvani and Alessandro Volta in cell and discovering a making stored electric energy available to human beings? [Refer to TB page 133 Q11] A. We should appreciate the efforts of Luigi Galvani and Volta for their discoveries for the development of mankind. Luigi Galvani: A biologist named Luigi Galvani from Italy once saw a frogs leg hung from a copper hook twitching violently when it touched another metal. Galvani did many more experiments with the legs of dead frogs. Galvani thought he had discovered living or biological electricity. Volta: Italian physicist is known for his pioneering work in electricity. He made the first electric battery called the voltaic cell. Among his many other contributions to science was the invention of a device known as the electrophorus which produced static charges. In honour to his work, Volt, the unit of electric voltage is named after him. SESSION 2. CHEMICAL EFFECTS OF ELECTRIC CURRENT AND ITS USES 59

—— CCE Based Practice Questions —— AS1-Conceptual Understanding Very Short Answer Type Questions 1. State true or false. [Refer to Session 9.1 ] (i) All materials conduct electricity equally. [] (ii) Distilled water is a good conductor of electricity. [] (iii) Distilled water which is a poor conductor of electricity, can be transformed into a good conductor of electricity. [] 2. Fill in the blanks. [Refer to Session 9.1 ] (i) We mostly get electric shocks with hands. (ii) The materials which allow current to pass through them are called of electricity. 3. State true or false. [Refer to Session 9.2 ] (i) A greenish blue spot is seen on the potato around the wire connected to the negative terminal of the battery. [] (ii) Dilute sulphuric acid is an electrolyte. [] CHAPTER 9. ELECTRICAL CONDUCTIVITY OF LIQUIDS 60

4. Answer the following questions in one sentence. [Refer to Session 9.2 ] (i) Give one use of electroplating. (ii) From the name of which famous scientist is the electrical term “voltage” derived? (iii) In electrolysis, which form of energy is chemical engery converted to? 5. Fill in the blanks. and plates [Refer to Session 9.2 ] (i) Alessandro Volta made his first cell using dipped in sulphuric acid. (ii) In electroplating, an metal is coated with a metal. Short Answer Type Questions 6. Answer the following questions in 3-4 sentences. (i) [(Session 9.1)] What are good conductors of electricity? (ii) [(Session 9.1)] Define electric conductivity. AS2-Asking questions and making hypothesis Short Answer Type Questions 7. Answer the following questions in 3-4 sentences. (i) [(Session 9.1)] Swathi has two materials in her hand. One is copper wire and other one is cotton thread. She wants to make a complete circuit. Which one of the men- tioned materials she is going to use to glow the bulb in the circuit and why? AS3-Experimentation and field investigation Short Answer Type Questions 8. Answer the following questions in 3-4 sentences. (i) [(Session 9.1)] Write an experiment to prove that normal water is a good conductor of electricity but distilled water is not. CHAPTER 9. ELECTRICAL CONDUCTIVITY OF LIQUIDS 61

Long Answer Type Questions 9. Answer the following questions in 6-8 sentences. (i) [(Session 9.1)] What is the procedure to check if a given liquid is a good conductor or a bad conductor of electricity? AS4-Information skills and projects Long Answer Type Questions 10. Answer the following questions in 6-8 sentences. (i) [(Session 9.1)] Collect information about different liquids which are used in our daily life. Prepare a table and classify them as good conductors or bad conductors of electricity. AS5-Communication through drawing and model making Short Answer Type Questions 11. Answer the following question. (i) [(Session 9.2)] Draw an experimental set up for the coating of iron key with copper by electroplating method. AS6-Appreciation and aesthetic sense, Values Long Answer Type Questions 12. Answer the following questions in 6-8 sentences. (i) [(Session 9.2)] Write a note on Luigi Galvani’s contribution to the field of electricity. AS7-Application to daily life, concern to bio diversity Short Answer Type Questions 13. Answer the following questions in 3-4 sentences. (i) [(Session 9.2)] Give some examples for the electroplated objects that we see in our daily life. CHAPTER 9. ELECTRICAL CONDUCTIVITY OF LIQUIDS 62

Objective Questions AS1-Conceptual Understanding 14. Choose the correct answer. (i) Which of the following liquid would conduct electricity? (A) Vegetable oil (B) Lemon juice (C)Distilled water (D) Honey (ii) Why do we use LED in the tester instead of bulb? (A) Because the LED glows only when the current is strong (B) Beacuse the LED costs lesser than the bulb (C)Because the LED glows even when the current is very weak (D)Because the LED is costlier than the bulb AS2-Asking questions and making hypothesis 15. Choose the correct answer. (i) Distilled water does not conduct electricity. What substance can be added to distilled water in small amounts to make it a good conductor of electricity? (A) Salt (B) Coconut oil (C) Alcohol (D)All of the above CHAPTER 9. ELECTRICAL CONDUCTIVITY OF LIQUIDS 63

AS3-Experimentation and field investigation 16. Choose the correct answer. (i) Electrolysis is the process of conversion of (A) electric energy to chemical energy (B) chemical energy to light energy (C)electric energy to light energy (D)chemical energy to electric energy (ii) A tester is used to check the conduction of electricity through two liquids, labeled A and B. It is found that the bulb of the tester glows brightly for liquid A while it glows verydimly for liquid B. You would conclude that (A) liquid A is a better conductor than liquid B (B) liquid B is a better conductor than liquid A (C)both liquids are equally conducting (D)conducting properties of liquids cannot be compared in this manner AS4-Information skills and projects 17. Choose the correct answer. (i) Following are some points to be remembered when electroplating a metal. Which of the following is an incorrect point? (A) The object to be electroplated should be free from greasy matter (B) The surface of the article should be smooth (C)The concentration of the electrolyte should be adjusted as to get smooth coating (D)Current must be same throughout CHAPTER 9. ELECTRICAL CONDUCTIVITY OF LIQUIDS 64

AS5-Communication through drawing and model making 18. Choose the correct answer. (i) In the following picture. The zinc and copper rods are called (A) electrolyte (B) electrodes (C) electrolysis (D) electroplating AS6-Appreciation and aesthetic sense, Values 19. Choose the correct answer. (i) Which metal is electroplated on iron for making cans used for storing food? (A) Tin (B) Copper (C) Zinc (D) Aluminium (ii) Which properties of chromium metal make it suitable for electroplating it on machinery parts? (A) It protects them from corrosion (B) It gives them a good polish (C)It protects them from scratches (D)All of the above CHAPTER 9. ELECTRICAL CONDUCTIVITY OF LIQUIDS 65

AS7-Application to daily life, concern to bio diversity 20. Choose the correct answer. (i) Which of the following options is not a use of electroplating? (A) To protect certain metals from corrosion (B) To give a good polish to the metal (C)To repair worn out parts of a machinery (D)To make the metal a good conductor CHAPTER 9. ELECTRICAL CONDUCTIVITY OF LIQUIDS 66

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1.2 Terminology i. Reflection – Reflection is the change in direction of a wave front at an interface between two different media so that the wave front returns into the medium from which it originated. ii. Reflection of light – When light rays travelling in a medium reach the boundary of some other medium, they turn back to the first medium. This phenomenon of turning back of light into the same medium after striking the boundary of an other medium is called Reflection of Light. iii. Incident ray – An incident ray is a ray of light that strikes a surface. iv. Reflected ray – The reflected ray, corresponding to a given incident ray, is the ray that represents the light reflected by the surface. v. Pinhole camera – A pinhole camera is a simple camera without a lens and with a single small aperture, a pinhole. vi. Inverted image – Image that shows the subject upside down when projected or seen vii. Blurred image – Image which is indistinct and hazy in appearance SESSION 1. LAWS OF REFLECTION AND FORMATION OF IMAGE BY A ... 68

1.3 Key Concepts i. A source of light, an opaque object and a screen are needed to form a shadow. ii. Light travels in a straight line. iii. When light gets reflected from a surface, the angle of reflection is equal to the angle of incidence. iv. The incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane. v. Principle of pinhole camera: Light rays from an object pass through a small hole to form an inverted image. vi. If we increase the size of the hole of the pinhole camera then we get a blurred image on the screen. vii. Light selects the path that takes the least time to travel. This principle was first given by Pierre de Fermat, a French lawyer and an amateur mathematician. viii. When light gets reflected from a surface, it selects the path that takes the least time. That is why the angle of incidence is equal to the angle of reflection. Q1. State the laws of reflection of light. [Refer to TB page 141 Q1] A. Laws of reflection of light are as follows. i. The angle of incidence is equal to the angle of reflection. ii. The incident ray, reflected ray and normal to the plane of reflection at point of reflection lie in the same plane. Q2. Explain the image formation by a pinhole camera with the help of a diagram. [Refer to TB page 141 Q4] A. With a pinhole camera, light is passed through a very small hole (pinhole), such as one made by a needle. When an object is exposed to sunlight or similar light, the light is reflected (scattered) in various directions. If we observe the object through the pinhole, the light which comes from the top of the object goes straight towards the bottom of the screen. Similarly the rays from the bottom of the object goes straight towards the top of the screen. This leads to the formation of an inverted image. SESSION 1. LAWS OF REFLECTION AND FORMATION OF IMAGE BY A ... 69

If we increase the size of the pin hole camera, we get a blurred image with big size. If the hole is very large, there will be a lot of light entering the pinhole camera, but no image will be seen at all. The size of the image x Increases on increasing the distance between the screen and pinhole. x Decreases on increasing the distance between the object and pinhole. x Increases on increasing the size of the object. Multiple Choice Questions 1. 'Angle of incidence = Angle of reflection' This rule is explained by ________ principle. [TB page 142 Q1] (a) Fermat (b) Newton (c) Archimedes (d) Pascal Correct Answer: a SESSION 1. LAWS OF REFLECTION AND FORMATION OF IMAGE BY A ... 70

SESSION 2 INTRODUCTION TO PLANE MIRROR AND REFLECTION OF LIGHT BY PLANE MIRRORS 2.1 Mind Map Formation of image by plane mirror 2.2 Terminology i. Plane mirror – A plane mirror is a mirror with a flat (planar) reflective surface. ii. Reflector – One that reflects, e.g.,– a polished surface for reflecting light or other radia- tion. iii. Clamp – A device made of wood or metal that is used to hold two things together tightly iv. Point object – A point object refers to a tiny object which is calculated or counted as a dot object to simplify the calculations. v. Object distance – The distance between the pole of the mirror and object is known as object distance. vi. Image distance – The distance between the pole of the mirror and image is known as image distance. vii. Plane of reflection – The plane in which the incident ray, reflected ray and normal lie is called the plane of reflection. SESSION 2. INTRODUCTION TO PLANE MIRROR AND REFLECTION OF ... 71

2.3 Key Concepts i. Silver metal is one of the best reflectors of light. ii. The thin layer of Silver or Aluminium is the reflecting surface at the backside of the mirror. iii. The plane in which the incident ray, reflected ray and normal lie is the plane of reflection. SESSION 2. INTRODUCTION TO PLANE MIRROR AND REFLECTION OF ... 72

Q1. Draw a ray diagram to understand the formation of an image for a pointed object by a plane mirror. Explain it. [Refer to TB page 142 Q10] A. In the following figure, O is a point object. Some rays from O reach the mirror and get reflected. When we look into the mirror, the reflected rays seem to be coming from the point I. So point I is the image of point object O. When we place an object in front of a mirror, we see an image of the same object in the mirror. The object is the source of the incident rays, and the image is formed by the reflected rays. In case of a plane mirror, the image formed is equal to the size of the object. Q2. If a ray incidents normally on a plane mirror, what will be the angle of reflection? [Refer to TB page 142 Q8] A. For a plane mirror: If the incident ray falls along the normal, the angle of incidence is 0 degree. Hence the angle of reflection is also 0 degree. Q1. Why can’t we see our image in a white sheet of paper though it reflects light? [Refer to TB page 142 Q14] A. Actually, paper is not flat. There are tiny rough spots that we cannot feel. The surface of the paper is not smooth, so it cannot reflect a cohesive image. Surface SESSION 2. INTRODUCTION TO PLANE MIRROR AND REFLECTION OF ... 73

roughness also contributes to diffuse reflection, causing the rays of light from different parts of a potential image to bounce back at different angles. Multiple and diffused reflections scatter the reflecting rays and all information about the image is lost. It scatters any reflected light, so we cannot see an image. Q1. How do you verify the 1st law of reflection of light with an experiment? [Refer to TB page 141 Q2] A. Learner’s activity (Learner can follow the given procedure). Aim – To verify the 1st law of reflection Material required –Mirror strip, drawing board, white paper, pins, clamps, scale and pencil Procedure – Take a drawing board and fix a white paper on it with the help of a clamp. Draw a straight line AB at the centre of the paper and also a normal (ON) to AB at the point ‘O’. Draw a straight line PQ making a certain angle (angle i) with ON as shown in the below figure. Fix two pins at the points P and Q on the paper vertically. Observe the image P1 of the pin P and Q1 of the pin Q in the mirror kept along the line AB. Fix two more pins R and S in such a way that they are in same line as that of P1 and Q1. Join R, S and O as shown in given figure. SESSION 2. INTRODUCTION TO PLANE MIRROR AND REFLECTION OF ... 74

Measure the angle between RS and ON (Angle of reflection). You will find that angle of incidence (i) = angle of reflection (r). Repeat the experiment for different angles of incidence and measure the corresponding angle of reflection. Record your observations in a table. S.No. i Is i=r r (Y/N) 1 30 degree 30 degree Y 2 40 degree 40 degree Y 3 45 degree 45 degree Y 4 50 degree 50 degree Y In all cases the angle of reflection is equal to the angle of incidence. Hence, the first law of reflection is verified. Q2. How do you verify the 2nd law of reflection of light with an experiment? [Refer to TB page 141 Q3] A. Learner’s Activity (Learners can follow the given procedure). Aim – To verify the 2nd law of reflection Material required – Mirror strip, drawing board, white paper, pins, clamps, scale and pencil Procedure – Take a drawing board and fix a white paper on it with the help of a clamp. Draw a straight line AB at the centre of the paper and also a normal (ON) to AB at the point ‘O’. Draw a straight line PQ making a certain angle (angle i) with ON as shown in the below figure. Fix two pins at the points P and Q on the paper vertically. Observe the image P1 of the pin P and Q1 of the pin Q in the mirror kept along the line AB. Fix two more pins R and S in such a way that they are in same line as that of P1 and Q1. Join R, S and O as shown in given figure. SESSION 2. INTRODUCTION TO PLANE MIRROR AND REFLECTION OF ... 75

Here, the line passing through P and Q is called incident ray and it touches the paper. The line joining R and S is called reflected ray and it touches the same paper. ON is the normal to the mirror at ‘O’ and it touches the same paper. Hence the incident ray, reflected ray and normal lie on the same paper. Hence, the second law of reflection is verified. Q3. Find the plane of reflection experimentally for the incident ray which passes through the heads of the pins pierced in front of the mirror. [Refer to TB page 141 Q5] A. Take a drawing board and fix a white paper on it with the help of a clamp. Draw a straight line AB at the centre of the paper and also a normal (ON) to AB at the point ‘O’. Draw a straight line PQ making a certain angle (angle i) with ON as shown in the below figure. Fix two pins at the points P and Q on the paper vertically. Observe the image P1 of the pin P and Q1 of the pin Q in the mirror kept along the line AB. Fix two more pins R and S in such a way that they are in same line as that of P1 and Q1. Join R, S and O as shown in given figure. SESSION 2. INTRODUCTION TO PLANE MIRROR AND REFLECTION OF ... 76

Adjust the pins so that they were pierced to the same height on the board. Put a paper on the head of the pins. The plane where the paper touches the mirror is called as the plane of reflection for the incident ray,which passes through the heads of the pins. Q1. In the adjacent figure, AO and OB are the incident and reflected rays respectively. Angle AOB = 90°. Find the values of the angle of incidence and angle of reflection? [Refer to TB page 142 Q11] A. Angle of incidence = Angle of reflection = 45° Q2. Hinduja stands in front of a plane mirror at a distance of 5 m from the mirror and observes her image in the mirror. If she moves 2 m towards the plane mirror, then what will be the distance between Hinduja and her image? [Refer to TB page 142 Q12] A. We know the distance between the object and the mirror is equal to the distance between the mirror and the image formed. SESSION 2. INTRODUCTION TO PLANE MIRROR AND REFLECTION OF ... 77

When Hinduja stands at a distance of 5 m from the mirror, the distance between the image formed and mirror is also 5 m. But when Hinduja moves 2 m forward, then the distance between Hinduja and mirror becomes 3 m. So the distance between mirror and image is 3 m. So, distance between Hinduja and image formed is 3+3= 6 m. Q3. The size of the image in the mirror seems to be decreased when you move the object towards your eye from the mirror. Draw a diagram showing the angles depicting the situation. [Refer to TB page 142 Q16] A. When we move the object from the mirror to our eye, the image in the mirror seems to move back in the mirror. Then the distance from the image to our eye increases. The angle made by the image at our eye is smaller than the angle made by the object. That is why the image looks smaller than the object. Here in the following image, O1 is the position of object initially and I1 is the image of the same. O2 is the position of the object when moved towards the eye and I2 is the image of that. Q1. Have you observed the image of the sky in the rain water pools on Earth? Explain the reflection of light in this context. [Refer to TB page 142 Q6] SESSION 2. INTRODUCTION TO PLANE MIRROR AND REFLECTION OF ... 78

A. Yes, in the context explained in the question, the rain water pools on Earth acts as a plane mirror. Look at the figure showing the reflection of light in a water pool on Earth. Let us assume that part of the sky is denoted by points O and P, as shown in the figure. Observe the incident rays from the parts of the sky and the reflected rays from the Earth denoted by the arrow heads on the light rays. The rays coming from the point O appears to be reflected from point I. So I is the image of the O. In a similar way rays from the P reflected from the water appears to be coming from point J. The rays coming from the middle part of OP form images between IJ. Thus IJ is the image of the part of the sky OP. So we can observe the image of the sky from the pool of rainwater present on Earth. Q2. Observe the adjacent figure. AB and BC are two plane mirrors arranged at 120°. A ray incidents at an angle 55°on AB. Find the values of 'x'? [Refer to TB page 142 Q15] A. We know that the first reflected ray makes an angle of 55° with the normal. Thus this ray makes an angle of 90°–55° = 35°with the horizontal. From the triangle made by the first reflected ray and the two mirrors, we see that the first reflected ray makes an angle of 25° with BC (because the sum of the interior angles of any triangle is 180°). Therefore, this ray makes an angle of 65° with the normal to BC. From the law of reflection, the second reflected ray makes an angle of 65°with the normal to BC. SESSION 2. INTRODUCTION TO PLANE MIRROR AND REFLECTION OF ... 79

So the value of x = 65° Multiple Choice Questions (1) A ray of light incidents on a plane mirror at an angle of 90°to its surface. What will be the angle of reflection? [TB page 143 Q3] (a) 0° (b) 90° (c) 450° (d)180° Correct Answer: a (2) If we move an object away from the plane mirror the size of the images seem to be [TB page 143 Q4] (a) increased (b) decreased (c) of the same size (d) image can’t be seen Correct Answer: b (3) An object is placed 7 cm distance from the plane mirror then distance of image is [TB page 143 Q6] (a) 3.5 cm (b) 14 cm (c) 7 cm (d) 21 cm Correct Answer: c SESSION 2. INTRODUCTION TO PLANE MIRROR AND REFLECTION OF ... 80

SESSION 3 CHARACTERISTICS OF AN IMAGE FORMED BY PLANE MIRROR AND ITS USES 3.1 Mind Map SESSION 3. CHARACTERISTICS OF AN IMAGE FORMED BY PLANE MIR... 81

3.2 Terminology i. Lateral inversion – Lateral inversion is the effect produced by a plane mirror in reversing images from left to right. ii. Virtual image – A virtual image is an image formed when the outgoing rays from a point on an object always diverge.The image appears to be located at the point of apparent divergence. A virtual image cannot be projected onto a screen. iii. Real Image – A real image is an image which is located in the plane of convergence for the light rays that originate from a given object. If a screen is placed in the plane of a real image the image will generally become visible on the screen. Examples of real images include the image seen on a cinema screen. 3.3 Key Concepts i. Image of a real object in a plane mirror is virtual, erect and of the same size as the object. ii. When we move away from the plane mirror, the image in a plane mirror appears to be small because of the small angle subtended at our eye. iii. Image in a plane mirror suffers lateral inversion. iv. A virtual image is an image formed when the outgoing rays from a point on an object always diverge. The image appears to be located at the point of apparent divergence. A virtual image cannot be projected on to a screen. v. A real image is an image which is located in the plane of convergence for the light rays that originate from a given object. If a screen is placed in the plane of a real image, the image will generally become visible on the screen. Examples of real images include the image seen on a cinema screen. SESSION 3. CHARACTERISTICS OF AN IMAGE FORMED BY PLANE MIR... 82

Q1. Why does the image in a plane mirror suffer lateral inversion? [Reflection on concepts [Refer to TB page 142 Q9] A. Viewing an object in a mirror is actually viewing it at a180° rotated state. This is the source of the lateral inversion. The light rays which come from the right side of the object get reflected from the plane mirror and reach our eye. Our brain feels that the ray(reflected ray) is coming from the inside of the plane mirror. That is why the right side of the object appears as the left side of the object. Q1. Collect information about 'Situations using plane mirror' and prepare a report. [Refer to TB page 142 Q17] A. Plane mirrors are useful in our daily life and for developing products using science and technology. Information about such uses are listed as below. x Plane mirrors are used to groom ourselves, comb our hair, shave, to put on make up and large mirrors are used as dressing mirrors. x Plane mirrors are used in the manufacture of solar cookers, rear view mirrors for vehicles, periscopes, kaleidoscopes and so on. x Mirrors are used in cloth shops, barber shops, jewellery shops and also for decoration of shops and buildings. Q1. Explain diagrammatically the image of the letter ‘B’ in a plane mirror. [Refer to TB page 142 Q13] A. SESSION 3. CHARACTERISTICS OF AN IMAGE FORMED BY PLANE MIR... 83

Q1. Discuss the merits and demerits of using mirrors in building elevation? [Refer to TB page 142 Q7] A. Merits of using mirrors: x They can be cut into different sizes. x They do not rust. x They do not let water to pass through them. Demerits of using mirrors: x They can break easily. x Using mirrors is more expensive. Multiple Choice Question (1) Which of the following letters does not suffer lateral inversion? [Refer to TB page 142 Q2] (a) C (b) O (c) B (d) N Correct Answer: b (2) Which of the following is incorrect with respect to the image in a plane mirror? [Refer to TB page 143 Q5] (a) image is erect (b) size of the image is same as the size of object (c) laterally inverted (d) image is real Correct Answer: d SESSION 3. CHARACTERISTICS OF AN IMAGE FORMED BY PLANE MIR... 84

—— CCE Based Practice Questions —— AS1-Conceptual understanding Very Short Answer Type Questions 1. Fill in the blanks. [Refer to Session 10.2 ] (i) A image cannot be projected onto a screen. (ii) The plane in which the incident ray, reflected ray and normal lie is called the . 2. State true or false. [Refer to Session 10.1 ] (i) A pinhole camera with a big hole will produce a crispy image on the screen. ] [ 3. Answer the following questions in one sentence. [Refer to Session 10.1 ] (i) If a ray incidents normally on a plane mirror, what will be the angle of reflection? 4. Fill in the blanks. [Refer to Session 10.1 ] (i) The first person who said that light always selects the path which takes the least time to travel between two points is . (ii) The incident ray, the normal at the point of incidence and the reflected ray all lie in the . Short Answer Type Questions 5. Answer the following questions in 3-4 sentences. (i) [(Session 10.1)] State the laws of reflection. CHAPTER 10. REFLECTION OF LIGHT AT PLANE SURFACES 85

(ii) [(Session 10.1)] What would happen if we increase the size of the hole of a pinhole camera? Long Answer Type Questions 6. Answer the following questions in 6-8 sentences. (i) [(Session 10.1)] Explain the process of formation of an image with a pinhole camera. Draw a ray diagram to show this. AS2-Asking questions and making hypothesis Short Answer Type Questions 7. Answer the following questions in 3-4 sentences. (i) [(Session 10.2)] Ritu asks her father why the word 'AMBULANCE' is written in reverse on the Ambulance Vehicle. What might her father have told her? (ii) [(Session 10.2)] What would you do to obtain the figures that are shown in figure 2 using a mirror strip and figure 1? AS3-Experimentation and field investigation 86 Long Answer Type Questions 8. Answer the following questions in 6-8 sentences. (i) [(Session 10.2)] Verify the laws of reflection experimentally. CHAPTER 10. REFLECTION OF LIGHT AT PLANE SURFACES

AS4-Information skills and projects Very Short Answer Type Questions 9. Answer the following questions in one sentence. [Refer to Session 10.2 ] (i) Analyse the table and write your conclusion. S.No. Angle of Angle of Is i=r? incidence of reflection of (Yes/No) 1 light (i) light (r) Yes 2 3 30° 30° 4 45° 45° Yes 60° 60° Yes 75° 75° Yes (ii) In the figure, AO and OB are the incident ray and reflected ray respectively. <AOB = 90° Find the values of the angle of incidence and angle of reflection? Long Answer Type Questions 10. Answer the following questions in 6-8 sentences. (i) [(Session 10.2)] Two plane mirrors are fixed at right angles to each other and an object is placed between them. Trace the path of the rays by which an observer sees the final image in one of the mirrors. CHAPTER 10. REFLECTION OF LIGHT AT PLANE SURFACES 87

AS5-Communication through drawing and model making Short Answer Type Questions 11. Answer the following question. (i) [(Session 10.2)] Draw a ray diagram to understand the formation of an image for a pointed object by a plane mirror. Long Answer Type Questions 12. Answer the following question. (i) [(Session 10.3)] Draw the image of the lateral inversion of the letter J. AS6-Appreciation and aesthetic sense, Values Short Answer Type Questions 13. Answer the following questions in 3-4 sentences. (i) [(Session 10.2)] Observe the figure. AB and BC are two plane mirrors arranged at 120°. A ray incidents at an angle of 55° on AB. Find the value of ‘x’. AS7-Application to daily life, concern to bio diversity Short Answer Type Questions 14. Answer the following questions in 3-4 sentences. (i) [(Session 10.3)] Make a solar heater/cooker using plane mirrors and write a report on the process of making it. CHAPTER 10. REFLECTION OF LIGHT AT PLANE SURFACES 88

Objective Questions AS1-Conceptual Understanding 15. Choose the correct answer. (i) If we increase the size of the hole of a pinhole camera, the image becomes (A) crispy. (B) focused. (C) blurred. (D)none of these (ii) The imaginary line which is perpendicular to the reflecting surface is the (A) plane of reflection. (B) normal. (C)incident ray. (D)reflected ray. (iii) According to the laws of reflection: (B) Incident angle >Reflected angle (A) Incident angle = Reflected angle (C)Reflected angle >Incident angle (D)Incident angle Reflected angle (iv) When light gets reflected from a surface, it selects the path that takes (A) many paths. (B) the least time. (C)a curved line. (D)none of these (v) A ray of light falls on a plane mirror with an incident angle of 35°. Select the angle of reflection. (A) 30° (B) 35° (C) 53° (D) 60° (vi) The image distance and the object distance in case of a plane mirror are (A) equal. (B) different. (C)sometimes equal. (D)none of these (vii) A ray of light incidents on a plane mirror at an angle of 90° to the reflecting surface. Select the angle of reflection. CHAPTER 10. REFLECTION OF LIGHT AT PLANE SURFACES 89

(A) 0° (B) 90° (C) 45° (D) 180° AS2-Asking questions and making hypothesis 16. Choose the correct answer. (i) Which of the following letters does not suffer lateral inversion? (A) C (B) O (C) B (D) N AS3-Experimentation and field investigation 17. Choose the correct answer. (i) When light gets reflected from a surface, the angle of reflection is equal to (A) 90°. (B) the angle of incidence. (C)the angle of normal to the reflecting plane. (D) 45°. (ii) The image formed by a plane mirror is always (A) virtual and erect. (B) virtual and inverted. (C)real and inverted. (D)real and erect. AS4-Information skills and projects 18. Choose the correct answer. (i) is used to deposit on glass plates instead of silver. (A) Iron (B) Nickel (C) Aluminium (D) Copper CHAPTER 10. REFLECTION OF LIGHT AT PLANE SURFACES 90

11. SOME NATURAL PHENOMENA SESSION 1 LIGHTNING - CAUSES, CHARGE, AND PROTECTION 1.1 Mind Map 1. Lightning SESSION 1. LIGHTNING - CAUSES, CHARGE, AND PROTECTION 91

2. Charge 1.2 Terminology i. Discharge – To relieve of a charge Or Electric discharge describes any flow of electric charge through a gas, liquid or solid. ii. Lightning – Lightning is defined as a flash of light that is caused by the discharge of electricity in the atmosphere. iii. Lightning conductor – A metal rod or wire fixed to an exposed part of a building or other tall structure to divert lightning harmlessly into the ground. iv. Crust – The outer layer of the Earth, between the surface and the mantle, which is upto 40 miles deep. v. Electroscope – An electroscope is an early scientific instrument that is used to detectthe presence and magnitude of electric charge on a body. 1.3 Key Concepts i. Lightning is caused due to the electric discharge between the earth and clouds or in between different clouds. ii. Lightning conductors are used to protect buildings. iii. The flow of charges through a conductor is called electric current. SESSION 1. LIGHTNING - CAUSES, CHARGE, AND PROTECTION 92

iv. Some objects can be charged by rubbing with other materials. v. Static electricity consists of charges that cannot flow. vi. Charges are of two types: positive and negative. vii. Like charges repel each other and unlike charges attract each other. viii. An electroscope is used to detect whether a body is charged or not. ix. Earthing is the process of transfer of a charge from a charged object to the earth. 1.4 Conceptual Understanding Q1. Sometime a cracking sound is heard while taking off our sweater during winters. Explain. [Refer to TB page 158 Q4] A. Woollen sweater gets charged due to friction with the body. When it is taken off, due to collision of charges a cracking sound is heard. Q2. Explain why a charged body loses its charge if we touch it with our hands? [Refer to TB page 158 Q5] A. If we touch a charged body it loses its charge because our body conducts its charge to earth. Q3. Suggest three measures to protect ourselves from lightning. [Refer to TB page 158 Q7] A. i. Stay in a completely closed place. If you are moving in a car, then remain there until the lightning is over. Close the windows of the car immediately. ii. Do not touch any electrical wires, telephone cables, metal pieces, etc. iii. Do not bathe in flowing water. This may cause electrical shock. Q4. Explain why a charged balloon is repelled by another charged balloon whereas an un- charged balloon is attracted by a charged balloon? [Refer to TB page 158 Q8] A. A balloon when charged acquires a particular type of charge. As both charged balloons are charged by the same process, both acquire same type of charge. Like charges repel each other and thus the two balloons repel each other. When an uncharged balloon is brought near to a charged balloon the uncharged body acquires opposite charge by the process of induction and thus gets attracted to the charged balloon. SESSION 1. LIGHTNING - CAUSES, CHARGE, AND PROTECTION 93

Q5. When does a piece of matter have a charge? [Refer to TB page 159 Q12] A. A piece of matter can be charged by: i. Rubbing with another object ii. Conduction iii. Induction Q6. What happens if two objects having same charge are brought close to each other? What happens if two objects having different charges are brought close? Can you give an example for this? [Refer to TB page 159 Q13] A. Like charges repel and unlike charges attract each other. Example: When two charged glass rods are brought closer to each other they repel as they both possess like charges, but when a charged glass rod and ebonite rods are brought closer they attract each other as they are oppositely charged. Q7. Give two examples of effects in your daily life which are caused by transfer of charges. [Refer to TB page 159 Q14] A. i. Sparks on electric pole when wires become loose or when wind blows and shakes the wires. ii. Earthing is provided in buildings to protect us from electrical shocks that may be caused due to any leakage of charge. iii. A lightning conductor provides an easy route for the transfer of electrical charge. Q8. Which of the following cannot be charged easily by friction? [Refer to TB page 158 Q1] (a) A plastic scale (b) A copper rod (c) An inflated balloon (d) A woollen cloth (e) Piece of wood A. (b) A copper rod SESSION 1. LIGHTNING - CAUSES, CHARGE, AND PROTECTION 94

Q9. When a glass rod is rubbed with a piece of silk cloth the rod: [Refer to TB page 158 Q2] (a) and the cloth both acquire positive charge. (b) becomes positively charged while the cloth has a negative charge. (c) and the cloth both acquire negative charge. (d) becomes negatively charged while the cloth has a positive charge. A. (b) becomes positively charged while the cloth has.a negative charge. Q10. Write True or False sentences among the following [Refer to TB page 158 Q3] (a) Like charges attract each other. A. False (b) A charged glass rod attracts a charged plastic straw. A. True (c) Lightning conductor cannot protect a building from lightning. A. False 1.5 Experimentation and Field Investigation Q1. Inflate two balloons and rub both of them with a cloth first and then with different material. Will they attract each other in both cases? [Refer to TB page 159 Q15] A. If two inflated balloons are rubbed with cloth, they get same charge and repel each other. If two inflated balloons rubbed with different material, they may get opposite charge and attract each other. SESSION 1. LIGHTNING - CAUSES, CHARGE, AND PROTECTION 95

1.6 Communication Through Drawing and Model Making Q1. Describe with the help of a diagram an instrument which can be used to detect a charged body. [Refer to TB page 159 Q19] A. The instrument that can be used to test whether an object is carrying charge or not is electroscope. Description of electroscope: Take an empty jam bottle. Take a piece of cardboard slightly bigger in size than the mouth of the bottle. Pierce a hole in it so that a metal paper clip could be inserted. Open out paper clip as shown in the figure. Cut two strips of aluminium foil about 4 cm×1 cm each and hang them on the paper clip. Insert the paper clip having the strips of aluminium foil in to the cardboard lid so that it is perpendicular to it as shown in the figure. This entire device works as an electroscope. Procedure to detect a charged body by using an electroscope: Charge a refill and make it touch the end of the paper clip. The aluminium foil repel each other because the strips of aluminium foil receive the same charge from the charged refill through the paper clip. The strips carrying similar charges repel each other and hence they move apart. This proves electroscope is useful in detecting a charged body. SESSION 1. LIGHTNING - CAUSES, CHARGE, AND PROTECTION 96

1.7 Application to Daily Life, Concern to Bio Diversity Q1. The weather department has predicted that a thunderstorm is likely to occur on a certain day. Suppose you have to go out on that day, would you carry an umbrella? Explain. [Refer to TB page 159 Q24] A. No, we should not carry an umbrella in a thunderstorm. During thunderstorms, electrical discharge from clouds can travel through the metallic rod of the umbrella. This may give an electrical shock to the person who is carrying it. Hence it is not safe to carry an umbrella during lightning. SESSION 1. LIGHTNING - CAUSES, CHARGE, AND PROTECTION 97

SESSION 2 EARTHQUAKE - CAUSES AND PROTECTION 2.1 Mind Map 1. Earthquake SESSION 2. EARTHQUAKE - CAUSES AND PROTECTION 98