Light Energy

Objective Questions

Question 1

Write true or false for each statement:

(a) The image formed by a plane mirror is real.

(b) When a light ray is reflected from a wall, the angle of incidence is not equal to the angle of reflection.

(c) The image of the right hand in a plane mirror looks like that of a left hand.

(d) The image formed by a plane mirror is upright.

(e) The image formed by a plane mirror can be obtained on a screen.

(f) The objects around us are seen due to irregular reflection of light.

(g) The speed of light in vacuum is 3 x 10⁸ m s^-1.

(h) A rose appears red in light of all the colours.

(i) A black paper absorbs light of all the colours and reflects none.

(j) The primary colours are red, blue and green.

Answer

(a) False
Correct Statement — The image formed by a plane mirror is virtual.

(b) False
Correct Statement — When a light ray is reflected from a wall, the angle of incidence is equal to the angle of reflection.

(c) True

(d) True

(e) False
Correct Statement — The image formed by a plane mirror cannot be obtained on a screen.

(f) True

(g) True

(h) False A rose appears red in white light. Correct Statement —

(i) True

(j) True

Question 2

Fill in the blanks:

(a) Angle of incidence = ...............

(b) The incident ray, the reflected ray and the normal lie ...............

(c) The image formed by a plane mirror is at a distance behind the mirror as ............... .

(d) The image formed by a plane mirror is ............... and ............... .

(e) We are able to see the objects around us due to ............... reflection.

(f) A ............... image cannot be obtained on a screen.

(g) One surface of mirror is made opaque by ............... it followed by a thin coating of paint of lead oxide.

(h) A plane mirror ............... reflect 100 percent light falling on it.

(i) The colour of an opaque object is the colour of light which it ............... .

(j) Magenta, cyan and yellow are the ............... colours.

Answer

(a) Angle of reflection

(b) in one plane

(c) the object is in front of it

(d) erect, virtual

(e) irregular

(f) virtual

(g) silvering

(h) does not

(i) reflects

(j) secondary

Question 3

Match the following:

Answer

Question 4a

A man standing in front of a plane mirror finds his image to be at a distance of 6 metre from himself. The distance of man from the mirror is:

1. 6 m
2. 3 m
3. 2 m
4. 12 m

Answer

3 m

Reason — The distance of image from the mirror = The distance of object from the mirror. Since the distance between the man and image is 6 m, so the distance between mirror and man is 3 m.

Question 4b

The angle between the incident ray and the ray reflected from the plane mirror is 70°. The angle of incidence will be:

1. 70°
2. 30°
3. 35°
4. 90°

Answer

35°

Reason — The angle of incidence = angle of reflection. Since angle between incident and reflected ray is 70°, so angle of incidence is 35°.

Question 4c

The image formed by a plane mirror is:

1. virtual and inverted
2. virtual and of same size
3. real and inverted
4. real and of same size

Answer

virtual and of same size

Reason — The image formed by a plane mirror is virtual and of same size as that of object.

Question 4d

The angle of incidence on a plane mirror is 30°. The angle of reflection will be:

1. 30°
2. 60°
3. 15°
4. 0°

Answer

30°

Reason — The angle of incidence = angle of reflection.

Question 4e

The angle of incidence on a plane mirror is 30°. The angle between the incident ray and the reflected ray is:

1. 30°
2. 15°
3. 60°
4. 90°

Answer

60°

Reason — The angle between the incident ray and the reflected ray = angle of incidence + angle of reflection, and the angle of incidence = angle of reflection.

Question 4f

The property due to which a light ray striking a surface is returned back into the same medium is called:

1. refraction
2. reflex action
3. reflection
4. regression

Answer

reflection

Reason — The property due to which a light ray striking a surface is returned back into the same medium is called reflection.

Question 4g

A ray of light after reflection from a mirror is known as:

1. reflected ray
2. normal
3. incident ray
4. refracted ray

Answer

reflected ray

Reason — A ray of light after reflection from a mirror is known as reflected ray.

Question 4h

The speed of light is maximum in:

1. glass
2. water
3. air
4. wood

Answer

air

Reason — The speed of light is maximum in air i.e. 3 x 10⁸ m s^-1.

Question 4i

A red rose is seen in green light. It will appear:

1. red
2. blue
3. yellow
4. black

Answer

black

Reason — A red rose absorbs the green light falling on it and reflects none, so it appears black.

Question 4j

The primary colours are:

1. Red, blue and yellow
2. Magenta, yellow and cyan
3. Red, Blue and cyan
4. Blue, green and red

Answer

Blue, green and red

Reason — Red, green and blue are the three primary colours.

Short/Long Answer Questions

Question 1

What do you mean by the term reflection of light?

Answer

The returning of light in the same medium after striking a surface is called reflection of light.

Question 2

How is a plane mirror made?

Answer

A plane mirror is made by silvering one side of a thin plane glass plate. The silvered surface is then coated with some opaque material so as to safeguard the silvering on it.

Question 3

Explain the following terms:

1. Incident ray

2. Reflected ray

3. Angle of incidence

4. Angle of reflection

5. Normal

Answer

1. The ray of light that strikes the surface is called the incident ray.

2. The ray of light which is returned back into the same medium after striking the reflecting surface is called the reflected ray.

3. The angle between the incident ray and the normal is called the angle of incidence. It is represented by the letter i.

4. The angle between the reflected ray and the normal is called the angle of reflection. It is represented by the letter r.

5. The perpendicular drawn on the surface at the point of incidence is called the normal.

Question 4

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 the angle of reflection r.

Answer

Below is the labelled diagram showing the reflection of a ray of light by a plane mirror:

Question 5

State the two laws of reflection of light.

Answer

The two laws of reflection of light are:

1. The angle of incidence is equal to angle of reflection.
2. The incident ray, the reflected ray and the normal all lie in the same plane.

Question 6

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

Answer

The laws of reflection of light can be verified by the following experiment:

1. Take a drawing board and a white sheet of paper. Fix the white sheet of paper on the drawing board. Draw a straight line MM₁ on the paper.
2. On the line MM₁ take a point O at its middle and draw a line OA such that ∠MOA is less than 90°. Draw a normal ON on MM₁ at point O.
3. Take a small plane mirror and place along the line MM₁. Use plasticine to fix the mirror on the paper so that it stands upright.
4. Fix pins P and Q vertically on the line OA. Set your eye on the other side of the normal ON along the direction as shown in below figure.
5. You will see images P^', Q^' of pins P and Q in the mirror.
6. Set your eye in line with the two images P^', Q^' and fix two more pins R and S on the paper in front of the mirror such that both the pins R and S are in line with P^', Q^'.

7. Remove the mirror and draw small circles around pins P, Q, R and S. Draw a line OB joining the points R and S.
8. AO is incident ray, OB is reflected ray and ON is normal at the point of incidence O.
9. Using a protractor measure angle of incidence i (∠AON), angle of reflection r (∠BON). We will find two angles are equal.

This experiment verifies the two laws of reflection:

1. Angle of incidence = Angle of reflection.
2. Incident ray, reflected ray and normal all lie in the same plane of paper which is the second law of reflection.

Question 7

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

Answer

The angle of incidence is 0°.

Question 8

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

Answer

Below diagram shows the reflection of a light ray incident normally on a plane mirror:

Question 9

The diagram 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.

Answer

(a) The angle of incidence is 15°.

(b) The angle of reflection is 15°.

Question 10

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

Answer

(a) Angle of incidence is 0°.

(b) Angle of incidence is 30°.

(c) Angle of incidence is 45°.

Question 11

The diagram 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.

Answer

Below diagram shows the reflected ray:

Law used to draw the direction of the reflected ray is:

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

Question 12

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

Answer

Normal ON is perpendicular on plane mirror, so

Angle of incidence = 90° - 30° = 60°.

Angle of reflection = 60°. (Since angle of incidence = angle of reflection)

Question 13

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

Answer

For an object in front of a plane mirror, the image is formed behind the mirror. The image is formed when the reflected rays are produced backwards. The image is as far behind the mirror as the object is in front of it.

Question 14

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

Answer

Below ray diagram shows the formation of image of a point object by a plane mirror:

Question 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 O is formed and seen by the eye.

Answer

The ray diagram showing formation of the image of point object O placed in front of a plane mirror is given below:

Question 16

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

Answer

The four characteristics of the image formed by a plane mirror are:

1. The image is upright or erect.
2. The image is virtual.
3. The image is of same size as the object.
4. The image is laterally inverted.

Question 17

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

Answer

The image formed is as far behind the mirror as the object is in front of it.

Question 18

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

(a) What is the distance of your image from the mirror?

(b) What is the distance between you and your image?

Answer

(a) The distance of the image from the mirror is 2 metre as the image formed is as far behind the mirror as the object is in front of it.

(b) The distance between object and image will be 4 m (object distance + image distance = 2 m + 2 m)

Question 19

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

Answer

The interchange of the left and right sides in the image of an object in a plane mirror is called lateral inversion.
Below diagram illustrates lateral inversion of an image:

Question 20

Write down the letter C and I as seen in a plane mirror?

Answer

Letter C and I will appear as shown below, when seen in a plane mirror:

Question 21

What is irregular reflection? Give an example.

Answer

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

Question 22

How do we see objects around us?

Answer

We are able to see the objects around us because of the diffused reflection of light from them in all directions.

Question 23

State two uses of a plane mirror.

Answer

The two uses of a plane mirror are:

1. It is used as a looking glass.
2. It is used in barber's shop for seeing the rear view at the back.

Question 24

Can light travel in vacuum?

Answer

Yes, light can travel in vacuum.

Question 25

State the speed of light in:

(a) air

(b) glass

Answer

(a) The speed of light in air is 3 x 10⁸ m s^-1.

(b) The speed of light in glass is 2 x 10⁸ m s^-1.

Question 26

State whether light slows down or speeds up in the following cases:

(a) Light going from air to glass.

(b) Light going from glass to water.

(c) Light going from water to air.

Answer

(a) When light goes from air to glass, it slows down.

(b) When light goes from glass to water, it speeds up.

(c) When light goes from water to air, it speeds up.

Question 27

What are primary colours? Name the three primary colours.

Answer

Primary colours are the colours of light by mixing which white light is obtained but they themselves cannot be obtained by mixing any other colours of white light. The three primary colours are red, green and blue.

Question 28

What are secondary colours? Name the three secondary colours.

Answer

Secondary colours are those colours which are obtained by mixing any of the two primary colours. Yellow, cyan and magenta are the three secondary colours.

Question 29

Fill in the blanks with the appropriate colour:

(a) Blue + ................ = Cyan

(b) Red + Blue + ..................... = White

(c) Red + Blue = ....................

(d) Green + Red = ........................

Answer

(a) Green

(b) Green

(c) Magenta

(d) Yellow

Question 30

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

Answer

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

Question 31

A rose appears red in white light. How will it appear in:

(i) green light,

(ii) red light?

Give a reason for each of your answers.

Answer

(i) A red rose appears black in green light because it absorbs the green light falling on it and reflects none.

(ii) A red rose appears bright red in red light because it reflects the red light falling on it and absorbs none of it.

Question 32

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

Answer

A piece of paper appears white in sunlight because it reflects light of all the colours. When the paper is viewed in red light it appears red colour because it reflects the red light falling on it.

Question 33

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

Answer

When seen in red light, the piece of paper will appear black only. This is because the piece of paper absorbs light of all colours therefore it appears black.