# Frank Solutions for Chapter 6 Light Class 9 Physics ICSE

### Exercise 6.1

1. What do you mean by light?

Light may be defined as the radiant energy which produces in us the sensation of sight. Light itself is invisible but makes other objects visible.

2. Is light a form of energy?

Yes, light is a form of energy that produces the sensation of vision in our eyes.

3. What is the velocity of light in vacuum?

The velocity of light in vacuum is 3 x 108 m/s.

4. Name two common sources of light.

Two sources of light are

1. Natural, for e.g. Sun
2. Artificial, for e.g. Light bulb.

5. Write down four characteristics of light.

Four characteristics of light are -

1. Light waves can travel through vacuum
2. Light waves are transverse waves
3. Wavelength of light waves is short so that their length is measured in centi-microns.
4. The velocity of light in vacuum is 3 x 108 m/s.

6. Name two luminous bodies.

The Sun and the stars are the two luminous bodies.

7. Name two non-luminous bodies.

Two non -luminous bodies are moon, chair.

8. Distinguish between convergent and divergent beams of light.

Difference between convergent and divergent beam of light.

9. What is meant by a ray of light?

A ray of light is the path along which light travels.

10. Give three distinctions between sound and light waves.

Three distinctions between light and sound waves are

1. Light waves can travel through vacuum while sound waves cannot.
2. Light waves are transverse waves while sound waves are longitudinal waves.
3. The velocity of light in air is 3 x 108 m/s while the speed of light in air is just about 330 m/s.

11. Identify the following kinds of beam of light.

(a) A ray of light

(b) Divergent beam of light

(c) Divergent beam of light

(d) Parallel beam of light

12. Name the substance through which light is made to pass.

The substance through which light is made to pass is called medium.

13. What is meant by rectilinear propagation of light?

Rectilinear propagation of light is that the light travels along a straight line.

14. Is glass an opaque medium?

No, glass is a transparent medium.

15. Are metals transparent by nature?

No, no metal is transparent by nature.

16. What is the difference between a ray of light and a beam of light.

Difference between a ray of light and a beam of light

17. Out of the following, chose the substance which is translucent medium:

Wood, water, waxed paper, stone, black paper.

Waxed paper is the translucent medium among the given substances.

18. Draw diagrams to illustrate the parallel, convergent and divergent beam of light.

Parallel beam of light is the beam in which rays of light travel parallel to each other.

Divergent beam of light is the beam of light in which rays of light diverge from each other as the beam progresses.

19. Give two common observations illustrating that light travels in a straight line path.

Two observations that proves that light travels in a straight line are :-

1. Sunlight coming through a hole in a dark room, we can easily see that light travels in a straight line
2. Light coming from a laser light, used for presentation, can also be seen to travel in straight line.

20. What do you mean by reflection?

When rays of light fall on a surface, they are turned back into the same medium in accordance with some definite laws. This process is known as reflection.

21. What is the cause of regular reflection?

A smooth and polished flat surface is the cause of regular surface.

22. State the laws of reflection.

Reflection obeys following two laws

1. The incident ray, the reflected ray, and the normal at the point of incidence, all lie in the same plane.
2. The angle of incidence and the angle of reflection are always equal.

23. What must be the minimum height of a plane mirror so that 1.6 m tall person can see his full image?

The height of plane mirror should be half of the size of the object to get a full image of the object. So for a man of height 1.6 m tall should use a 0.8 m tall plane mirror.

24. What is meant by rectilinear propagation of light?

Rectilinear propagation of light is that the light travels along a straight line.

25. What is meant by lateral inversion?

Lateral inversion is the reversal of image experienced in a plane mirror. The image is of the same size and equidistant from the object but the left and right sides are transposed.

26. Name one phenomenon which is based on the laws of reflection.

Formation of image is the phenomenon based on laws of reflection.

27. Name the principle employed in a periscope.

The principle employed in a periscope is successive reflections from two plane mirrors.

28. What do you mean by the point of incidence?

The point at which the light is incident on the reflecting surface is called the point of incidence.

29. What do you mean by a mirror?

Any smooth, highly polished reflecting surface is called mirror.

30. What is meant by a plane mirror?

A smooth, highly polished plane surface is called plane mirror.

31. Name the angle between the incident ray and the normal to the surface at the point of incidence.

The angle between the incident ray and the normal to the surface at the point of incidence is called the angle of incidence.

32. The angle of incidence is 20°. What is the angle between the incident and the reflected rays?

Angle of reflection to the normal = angle of incidence to the normal = 20°

So the angle between incident ray and reflected ray = Angle of incidence +

Angle of reflection = 20 + 20 = 40°

33. The angle between an incident ray and the mirror is 35°.

(i) What is the angle of incidence?

(ii) What is the angle of reflection?

(iii) What is the total angle turned by the ray of light?

(iv) What is the angle between the incident and the reflected rays?

Given, angle between incident ray and mirror = 35°

(i) Angle of incidence = angle of mirror to the normal - angle between incident ray and mirror

= 90 - 35

= 55°

(ii) Angle of reflection = angle of incidence = 55°

(iii) Total angle turned = angle of incidence + angle of reflection

= 55 + 55

= 110°

(iv) The angle between incident ray and reflected ray = Angle of incidence + Angle of reflection

= 55 + 55

= 110°

34. A boy is standing in front of a plane mirror at a distance of 3 m from it.

(i) What is the distance between the boy and his image?

(ii) If the boy moves 1 m backward, find the distance between the image and the boy.

Given, distance of boy from the mirror = 3m

(i) Distance of image from mirror = distance of boy from the mirror = 3m

Distance between boy and his image = distance of boy from the mirror + distance of image from mirror = 3 + 3 = 6 m

(ii) Now, distance of boy from the mirror = 4m

Distance of image from mirror = 4m

Distance between boy and his image = distance of boy from the mirror + distance of image from mirror = 4 + 4 = 8 m.

35. The reflection of a ray of light (OA) normally incident on a plane mirror is shown below.

What are the angles of incidence and reflection?

The angle of incidence = angle of reflection = 0° because the image is perpendicular to the surface of the plane mirror.

36. State two main characteristics of the image formed by a plane mirror.

Two characteristics of image formed by plane mirror are

1. Image is erect and virtual
2. Image and object are of same size.

37. Draw two sets of rays of light to show the formation of image of the letter P as shown in fig. 15.

The answer diagram is

The reflection takes place at the surface of the plane mirror in accordance with the laws of reflection in which the angle of incidence is equal to angle of reflection.

The light coming from the letter P is reflected from the surface of plane mirror, When these reflected rays are produced backwards, they form an inverted virtual image of letter P which is same in size of letter P.

38. The answer diagram is:

The answer diagram is:-

39. Fig. 19 shows an incident ray AO and the normal ON on a plane mirror. Draw the reflected ray and then find the angle between the incident and reflected rays.