 Class: 7

Subject: Physics

Chapter: 4/Light Energy

Date: 15/08/21

1. What do you mean by the term reflection of light?

Ans: When light strikes a polished surface it comes back in the same medium, is called reflection of light.

1. How is a plane mirror made?

Ans: To make a plane mirror, a thin piece of glass is taken. One surface of the mirror is made opaque by silvering it. On the top of that another thin coating of red lead oxide is given which protects the silvering of the mirror.

1. Explain the following terms:

Incident ray: The ray of light falling on the surface AB is called the incident ray. In the following figure PN is incident ray.

Reflected Ray: The incident ray bouncing back in the same medium after striking the reflecting surface is called a reflected ray. In the following figure and q is the reflected ray.

Angle of Incidence: The angle formed between the incident ray and the normal is the angle of incidence (PNMis the angle of incidence.)

Angle of reflection: The angle formed between the normal and the reflected ray is called angle of reflection (MNQ is the angle of reflection).

Normal: It is the line drawn perpendicular to the reflecting surface at the point of incidence. MN is normal.

1. Draw a diagram showing the reflection of a light ray from a plane mirror. Label on it the incident ray, the reflected ray, the normal, the angle of incidence i  and angle of reflection r.

Ans:

PN is the incident ray

NQ is the reflected ray

NM is normal

<PNM is the angle of incidence i

<MNQ is the angle of reflection r

1. State the two laws of reflection of light.

Ans: The 2 laws of reflection of light are:

1. The incident ray, normal and the reflected ray all lie in the same plane.

2. The angle of incidence is equal to the angle of reflection.

6. Describe an experiment to verify the      laws of reflection of light.

Ans: The incident ray, normal and the reflected ray all lie in the same plane.

The angle of incidence is equal to the angle of reflection. <i=<r

Verification

Take a wooden drawing board and fix a white sheet of paper on it. In the middle of the paper draw straight line KK' mark a point B on it. Draw a perpendicular BN. Play some mirror XX' online KK' such that polished side of mirror is along the line. Hold the mirror in the mirror holder.

Fix two Steel pins P and Q on the straight line AB. Look for the images of the pins P and Q antics two pins P'Q'  such that P'Q' animations of P and Q are all in the same straight line.

Remove the pins and draw small circles around the pin pricks.

Remove the mirror also. Join P'Q' and produce the straight line to meet at B.

Measure <ABN= i and <CBN= r

It is found that< i= <r. This proves that angle of incidence is equal to angle of reflection.

As a incident ray, the reflected ray and the normal lie in the plane of paper, therefore, they lie in the same plane.

7. A ray of light falls normally on a plane mirror. What is the angle of incidence?

Ans: Angle of incidence is 0°. Since angle of incidence is the angle between incident ray and normal. Direction of reflected ray is along BA opposite to the direction of incident ray.

8. Draw a diagram to show the reflection of a light ray incident normally on a plane mirror.

Ans:

PN is the incident ray

NQ is the reflected ray

NM is normal

<PNM is the angle of incidence i

<MNQ is the angle of reflection r

9. The diagram in figure 4.18 shows an incident ray AO and the reflected ray OB from a plane mirror. The angle AOB is 30°.  Draw normal on the plane mirror at the point O and find:

a.  the angle of incidence

b. the angle of reflection.

Ans:

ON is normal on the plane mirror at point o.

ON is perpendicular on a plane mirror

Angle of incidence<i =<AON and the angle of reflection <BON

Since,<i=<r

<AOB =30°

or, <AON+<BON = 30°

or, <i+<I=30°

or,2<i=30°

or, <i=30/2=15°

So, Angle of incidence=<i=15°

And Angle of reflection=<r=15°

10. In the following diagrams measure and write the angle of incidence and draw the reflected ray in each case.

Ans:

11. The diagram in figure 4.20 shows an incident ray AO  and the normal ON on a plane mirror. Draw the reflected ray. State the law you use to draw the direction of the reflected ray.

Ans:

Law of reflection of light is used to draw the direction of the reflected ray. This law states that angle of incidence is equal to the angle of reflection. <i=<r

12. The following diagram shows an incident ray and the normal o n on a plane mirror. Find the angle of incidence and the angle of reflection.

Ans:

13. State in words, how do you find the location of image of an object formed by a plane mirror.

Ans: The location of image of a point object is as far behind the mirror as the object is in front of it.

14. Draw ray diagram showing the information of image of a point object by a plane mirror.

Ans:

15. The following diagram shows a point object O placed in front of a plane mirror. Take two rays from the point O and show how the image of is formed and seen by the eye.

Ans:

16. State four characteristics of the image formed by a plane mirror.

Ans: The 4 characteristics are:

1. The images is upright or erect.

2. The image is virtual.

3. The image is laterally inverted.

4. The images of same size as the object.

17. How is the position of image formed by a plane mirror related to the position of the object?

Ans: The image formed by a plane mirror is laterally inverted, upright, of the same size and is formed far behind the mirror as the object is in front of it.

18. You are standing at a distance 2 metre from a plane mirror.

1. What is the distance of your image from the mirror?

2. What is the distance between you and your image?

Ans: a. Distance of image from the mirror is also 2 metre.

b. Distance between me and my image is 4 meter.

19. What is meant by lateral inversion of an image in a plane mirror? Explain it with the help of a diagram.

Ans: Interchange of size between the object and its image is called lateral inversion.

Example:

20. Write down the letter C and I are seen in a plane mirror.

Ans: ) and I

21. What is irregular reflection? Give an example.

Ans: When a beam of light falls on such a surface which is not perfectly smooth and polished such as wall, wood, paper etc the different portions of the surface reflect light in different directions. Such a reflection of light in different directions from an uneven surface is called the regular or diffused reflection.

22. How do we see objects around us?

Ans: Objects around us are seen when light after striking them, returns in the same medium and reached our rice.

23. State two uses of a plane mirror.

Ans: It is used as a looking glass and in periscope.

24. Can light travel in vacuum?

Ans: Yes, light can travel in vacuum or air.

25. State the speed of light in (a) air, (b) glass.

Ans: a. Air- 3 ×10⁸

b. Glass-2×10⁸

26. State whether light slows down low speeds up in the following cases:

1. Light going from air to glass.

2. Light going from glass to water.

3. Light going from water to air.

Ans: a. Slows down

b. Speeds up

c. Speeds up

27. What are primary colours? Name the three primary colours.

Ans: primary colours are colours of light by mixing which white light is obtained. The three primary colours are red, green and blue.

28. What are secondary colours? Name the three secondary colours.

Ans: secondary colours are the colours of light which are obtained by mixing the two primary colours. The 3 secondary colours yellow, cyan and magenta.

29. Fill in the blanks with appropriate colour:

Ans: a. Green

b. Green

c. Magenta

d. Yellow

30. The leaves appear green when seen in white light. Give a reason.

Ans: Leaves appear green in white light because they reflect only the green light and absorb the light of all the other colours.

31. Eros appears red in white light. How will it appear in i. green light ii. red light? Give reason for each of your answers.

Ans: i.If a red rose is seen in green light, it appears black. The reason is that the rose absorbs the green light falling on it and reflects none.

ii. If a red rose is seen in red light it appears bright red. This is because the rose reflects the red light falling on it and  absorbs none of it.

32. Why does a piece of paper appears white in sunlight? How would you expect it to appear when viewed in red light?

Ans: A piece of paper appears white in sunlight because it reflects light of all the colours. It would appear red when viewed in red light.

33. A piece of paper appears black in sunlight.  What will be its colour when seen in red light?

Ans: A piece of paper appear black in sunlight. It would appear black when seen in red light because it absorbs light of all the colours.

Class: 7

Subject: Physics

Chapter: 3/Energy

1. Define the term energy.

Ans: Energy is defined as the capacity or ability of doing work.

1. State the unit of energy and define it.

Ans: The energy is measured in the same unit as work. Therefore the SI unit of energy is joule (symbol J).

A body is said to possess an energy of 1 joule if a force of 1 Newton moves the body by a distance of 1 m in the direction of force. Another unit of energy is calorie (symbol cal) where 1cal=4.2 J. A bigger unit is kilo -calorie (symbol kcal) where 1 kcal= 1000 cal.

1. Name the different forms of energy.

Ans: The different forms of energy are: a. Mechanical energy

b. Heat energy

c. Light energy

d. Chemical energy

e. Sound energy

f. Magnetic energy

g. Electrical energy and

h. Atomic energy or nuclear energy.

1. What are the two kinds of mechanical energy.

Ans: The two kinds of mechanical energy are potential energy and kinetic energy.

1. What is potential energy? State its unit.

Ans: Potential energy of a body is the energy possessed by it due to its state of rest or position. It is the energy stored when work is done on the body to bring it to that state or position. It is written as P.E.or U.

The SI unit of potential energy is joules.

1. Give an example of a body that has potential energy, in each of the following: (I) due to its position,

Ans: Water at a height has potential energy stored in IT. Falling water from a height can be used to do work like turning a wheel.

(ii) due to its state.

Ans: a stretched rubber band (elongated state) has potential energy.

1. State two factors on which the potential energy of a body at a certain height above the ground depends.

Ans: The potential energy of a body in the raised position depends upon the following two factors:

1. The mass of the body: Greater the mass of the body, greater is the potential energy of the body.

2. Its height above the ground: Higher the height of the body, greater is its potential energy.

8. Two bodies A and B of masses 10kg     and 20kg respectively are at the same height above the ground. Which of the two has greater potential energy?

Ans: The body B having mass 20kg has the greater potential energy.

9. A bucket full of water is on the first floor of your house and another identical bucket with same quantity of water is kept on the second floor. Which of the two has greater potential energy?

Ans: The bucket full of water kept on the second floor has the greater potential energy.

10. Define the term kinetic energy. Give one example of a body which possesses kinetic energy.

Ans: Kinetic energy of a body is energy possessed by its due to its motion. It is the energy stored when work is done to bring the body in motion. In a swinging pendulum moving to and fro, the bob has the kinetic energy.

11. State two factors on which the kinetic energy of a moving body depends.

Ans: Kinetic energy and of a moving body depends on two factors:

1. On the mass of the body ( greater the mass of the body, greater is its kinetic energy),

2. On the speed of the body ( more the speed of the body, more is its kinetic energy).

12. Two toy cars A and B of masses 500 gram and 200 gram respectively are moving with the same speed. Which of the two has greater kinetic energy?

Ans: The toy car a having mass 500 gram has greater kinetic energy.

13. A cyclist doubles his speed. How will his kinetic energy change: increases,decreases or remains the same?

Ans: When a cyclist doubles his speed. His kinetic energy increases.

14. Name the form of energy which a wound up watch spring possesses.

Ans:A wound up watch spring has the potential energy because of its wound up state. As a spring in winds itself, the potential energy changes into the kinetic energy. This kinetic energy does work in moving the arms of the watch.

15. Can a body possess energy even when it is not in motion? Explain your answer with an example.

Ans: Yes, a body possesses energy even when it is not in motion. Consider a body raised to a certain height h. Its kinetic energy will be zero but the body will have potential energy stored in it.

16. Name the type of energy (kinetic or potential) possessed by the following:

i) a moving cricket ball.

ii) a stone at rest on the top of a building.

III) compressed spring.

iv) a moving bus.

v) a bullet fired from a gun.

vi) water flowing in a river.

vii) a stretched rubber band.

Ans: i). Kinetic energy

ii. Potential energy

III. Potential energy

iv. Kinetic energy

v. Kinetic energy

vi. kinetic energy

vii. Potential energy

17. Give one example to show the conversion of potential energy to kinetic energy when put in use.

Ans: A hammer at the height has potential energy stored in it. The hammer has this energy because of its position at the height. In the figure below the hammer is dropped on a nail fixed on a piece of wood which has the kinetic energy. Thus, the potential energy stored in the hammer in its raised position changes into the kinetic energy when the hammer dropped. This kinetic energy does work on the nail as the hammer strikes the nail as it makes the nail to move into the wood.

18. State the energy changes that occur in the following:

i. The unwinding of a watch spring.

ii. Burning coal while operating a steam engine.

III.  Lighting of a torch bulb.

iv. An electric generator or dynamo.

Ans: i. Potential energy to kinetic energy.

ii. Chemical energy of coal changes to heat energy of the steam. Heat energy changes into mechanical energy.

III. Chemical energy into light energy and heat energy.

iv. Electrical energy change into mechanical energy.

19. Energy can exist in several forms and may change from one phone to another. Give two examples to show the conversion of energy from one form to another.

Ans.: The examples that show the conversion of energy from one form to another are:

1. In a steam engine, the chemical energy of the coal first changes into the heat energy of the steam. Then heat energy of steam changes into the mechanical energy which makes the train to move.

2. In an electric motor or in fan the electrical energy changes into the mechanical energy. This energy that is the axle of motor or the blades of the fan.

20. Give one relevant example for each of the following transformation of energy:

1. Electrical energy to heat energy.

2. Electrical energy to mechanical energy.

3. Electrical energy to light energy.

4. Chemical energy to heat energy.

5. Chemical energy to light energy.

Ans: I. In an electric heater the electrical energy changes into heat energy.

II. An electric generator

III. Lighting of a bulb.

IV. Burning of wood

V. Burning of firecrackers.

21. What do you mean by conservation of mechanical energy? State the condition when it holds.

Ans: The conservation of mechanical energy means the  total mechanical energy is equal to the sum of kinetic energy and potential energy.

The mechanical energy is conserved when there are no frictional forces. In other words the mechanical energy is conserved strictly in vacuum where fiction due to air is absent.

22. Give an example to show that the sum of potential energy and kinetic energy remains constant if function is ignored.

Ans: During the vertical fall of ball, if friction due to air is neglected, the total sum of potential energy and kinetic energy at each point of its path remains same.

23. A ball is made to fall freely from a height. State the kind of energy possessed by the ball when it is

a. at the highest point

b. just in the middle

c. at the ground.

Ans: a. Potential energy

b. Potential energy plus kinetic energy

c. Kinetic energy

24. State the changes in form of energy while producing hydroelectricity.

Ans: The water in motion in a river or sea has the kinetic energy. The flowing water of river is collected in a dam at a high altitude. The water stored in the dam has the potential energy. When water from dam falls on the water turbine and the potential energy of the water stored in dam changes into its kinetic energy and this kinetic energy of water is transferred to the blades of turbine as the kinetic energy which rotates the turbine. As the turbine rotates, it rotates the armature of the generator or dynamo to produce electricity.

25. A truck starts from rest on a plane road. What are the possible energy changes taking place while the truck is in motion?

Ans: Chemical energy to mechanical energy.                              