ICSE Solutions for Chapter 4 Light Energy Class 7 Physics Selina Publisher

Test Yourself


A. Objective Questions

1. Write true or false for each statement

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

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

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

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

(c) The image formed by a plane mirror can be obtained on a screen.
Answer
False
The image formed by a plane mirror cannot be obtained on a screen.

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

(g) The speed of light in vacuum is 3×108 ms-1
Answer
True

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

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

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

2. Fill in the blanks
(a) Angle of incidence = _______
(b) The incident ray, the reflected ray and the normal lie in one ______
(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 erect 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) plane 
(c) the object is in front of it. 
(d) virtual.
(e) irregular 
(f) virtual 
(g) silvering 

3. Match the following

Column A

Column B

(a) A light ray passes from air to glass

(i) speeds up

(b) A light ray passes from glass to water

(ii) reflects red light

(c) virtual image

(iii) primary colours

(d) Red rose

(iv) plane mirror

(e) Red, green and blue

(v) slows down

Answer

Column A

Column B

(a) A light ray passes from air to glass

(v) slows down

(b) A light ray passes from glass to water

(i) speeds up

(c) virtual image

(iv) plane mirror

(d) Red rose

(ii) reflects red light

(e) Red, green and blue

(iii) primary colours

4. Select the correct alternative 

(a) 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
2. 3 m 

(b) 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
3. 35°

(c) 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 
2. virtual and of same size

(d) 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 
1. 30°

(e) 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 
3. 60°

(f) 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
3. reflection

(g) A ray of light after reflection from a mirror is known as
1. reflected ray
2. normal
3. incident ray
4. refracted ray
Answer
1. reflected ray

(h) The speed of light is maximum in
1. glass
2. water
3. air
4. wood
Answer 
3. air 

(i) A red rose is seen in green light. It will appear.
1. red
2. blue
3. yellow
4. black
Answer 
4. black 

(j) 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
4. Blue, Green and Red 

B. Short/Long Answer Questions


1. What do you mean by the term reflection of light ?
Answer
Reflection of light: When light strikes a polished surface it comes back in the same medium, is called reflection of light.

2. How is a plane mirror made ?
Answer
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.

3. Explain the following terms:
Incident ray, Reflected ray, Angle of incidence, Angle of reflection, Normal.
Answer
  • Incident ray: The ray of light falling on the surface AB is called the incident ray. In figure PN is the incident ray.
  • Reflected ray: The incident ray bouncing back in the same medium after striking the reflecting surface is called reflected ray. In figure NQ is the reflected ray.
  • Angle of incidence: The angle formed between the incident ray and the normal is the angle of incidence. (PNM is 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 the normal.

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 
AO is the incident ray
OB is the reflected ray

ON is the normal
∠AON is the angle of incidence
∠NOB is the angle of reflection.

5. State the two laws of reflection of light.
Answer
Laws of reflection:
  • 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.

6. Describe an experiment to verify the laws of reflection of light. Ans. Laws of reflection.
Answer
(i) The incident ray, the reflected ray and the normal at the point of incidence, lie in the
same plane.
(ii) The angle of incidence and angle of reflection are equal i.e. ∠i = ∠r.
Verification
  • Take a wooden drawing board and fix a white sheet of paper on it. In the middle of paper draw a straight line KK. Mark a point B on it. 
  • Draw a perpendicular BN. Place a mirror XX’ on line 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 atleast 10 cm apart. 
  • Look for the images of the pins P and Q and fix two pins P Q’ such that P’, Q’ and images 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 the 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 ?
Answer
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.
Answer
AO is the incident ray
OB is the reflected ray

ON is the normal
∠AON is the angle of incidence
∠NOB is the angle of reflection.

9. The diagram in Fig. 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 :
Answer
(i) the angle of incidence
(ii) the angle of reflection
ON is normal on the plane mirror at point O
ON is perpendicular on a plane mirror
Angle of incidence ∠i = ∠AON
and angle of reflection ∠r = ∠BON
Since, ∠i – ∠r
∠AOB = 30°
⇒ ∠AON + ∠BON = 30°
⇒ ∠i + ∠i – 30°
⇒ 2 ∠i =30°
⇒ ∠i = 30/2 = 15°
∴ Angle of incidence = ∠i = 15° and Angle of reflection ∠i = 15°

10. In the following diagrams, measure and write the angle of incidence and draw the reflected ray in each case.
Answer
(a) ∠i = 90°
∠r = 90°

(b) ∠i = 30°
∠r = 30°
(c) ∠i = 30°
∠r = 30°


11. The diagram in fig. 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 

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 AO and the normal ON on a plane mirror. Find the angle of incidence and angle of reflection.
Answer
ON is perpendicular on a plane mirror
Angle of incidence ∠i (∠AON) i.e.
Angle between incident ray and normal ray = 90° – 30° = 60°
Angle of Reflection = 60°
∴ ∠i = ∠r
∴ Angle between incident and reflected ray i.e. ∠AOB = 60 + 60 = 120°

13. State in words, how do you find the location of image of an object formed by a plane mirror.
Answer
The location of image of a point object is as far behind the mirror as the object is in front of it.

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

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 

16. State four characteristics of the image formed by a plane mirror.
Answer 
  • The image formed is erect.
  • The image is of the same size as that of the object.
  • The image is laterally inverted. Right side appears to be left and left side appeared to be right.
  • The image formed behind the mirror is at equal distance as the object is in front of the mirror.

17. How is the position of image formed by a plane mirror related to the position of the object?
Answer
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.
(a) What is the distance of your image from the mirror ?
(b) What is the distance between you and your image ?
Answer
(a) Distance of image from the mirror is also 2 metre.
(b) Distance between me and my image is 4 metre.

19. What is meant by lateral inversion of an image in a plane mirror ? Explain it with the help of a diagram.
Answer
Lateral Inversion: Inter change of sides between the object and its image is called Lateral Inversion.
Example:

It means image formed behind the mirror is as far behind the mirror as object is in front of it.
i.e. distance of M = distance of M in distance of O in front of mirror = distance of O image, and so on.

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

21. What is irregular reflection? Give an example.
Answer
Irregular reflection: 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. Such a reflection of light from an uneven surface is called the irregular or diffused reflection.

22. How do we see objects around us ?
Answer
Objects are seen when light after striking them, returns in the same medium and reach our eyes.

23. State two uses of a plane mirror.
Answer 
It is used as :
(i) Looking glass
(ii) In periscopes.

24. Can light travel in vacuum ?
Answer
Yes, light can travels in vacuum or air, a distance of nearly 299,792,458 metre (or nearly 3×108 metre) in one second. Thus, the speed of light in vacuum (or air) is 3×108 ms-1 nearly.

25. State the speed of light in (a) air, (b) glass.
Answer 
(a) Air: 3×108 ms-1
(b) Glass: 2×108 ms-1

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) Slows down
(b) Speeds up
(c) Speeds up

27. What are the primary colours ? Name the three primary colours.
Answer
Primary colours are the colours of light by mixing which white light is obtained. They are:
(i) red
(ii) green and,
(iii) blue.
Red + Green + Blue = White

28. What are the secondary colours? Name the three secondary colours.
Answer
Secondary colours are the colours of light which are obtained by mixing the two primary colours. They are (i) yellow, (ii) cyan, and (iii) magenta.

29. Fill in the blanks with the appropriate colour
(a) Blue + ______ = Cyan
(b) Red + Blue + ______ = Vhite
(c) Red + Blue = ______
(d) Green + Red = ______
Answer
(a) Blue + Green = Cyan
(b) Red + Blue + Green = White
(e) Red + Blue = Magenta
(d) Green + Red = Yellow

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 the other colours.

31. A rose appears red in white light. How will it appear in (i) green light, (ii) red light? Give a reason for your answer for each.
Answer
(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 reflect 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 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. It would appear red when viewed in red light.

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