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Std 8 General Science · Chapter 16

16. Reflection of Light

Complete solutions to the Exercises. Maharashtra State Board (Balbharati), Grade Eight General Science.

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Exercises

1Fill in the blanks.

(i) The perpendicular to the mirror at the point of incidence is called the normal.

(ii) The reflection of light from a wooden surface is irregular reflection.

(iii) The working of Kaleidoscope is based on the properties of reflection of light.

2Draw a figure describing the following. The reflecting surfaces of two mirrors make an angle of 90° with each other. If a ray incident on one mirror has an angle of incidence of 30°, draw the ray reflected from the second mirror. What will be its angle of reflection?
30°30°60°60°O₁O₂Incident rayFinal reflected rayMirror 1Mirror 2The two mirrors areat 90° to each othernormalnormal

A ray reflected in turn by two mirrors placed at 90° to each other.

At the first mirror, ∠i = ∠r = 30°.

The reflected ray falls on the second mirror at 90° − 30° = 60° from the normal.

So, the angle of reflection at the second mirror is 60°.

3How will you explain the statement ‘we cannot see the objects in a dark room’?

We can see an object only when light reflected from it enters our eyes. In a dark room, no light falls on the objects. So no light is reflected from them. Hence, we cannot see the objects in a dark room.

4Explain the difference between regular and irregular reflection of light.
Regular reflectionIrregular reflection
It takes place from a smooth surface.It takes place from a rough surface.
The angles of incidence of all rays are equal.The angles of incidence are different at different points.
The reflected rays are parallel.The reflected rays are not parallel.
A clear image is formed.A clear image is not formed.
Example: Plane mirror.Example: Wooden surface.

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5Draw a figure showing the following: (a) Incident ray (b) Normal (c) Angle of incidence (d) Angle of reflection (e) Point of incidence (f) Reflected ray.
riA — Incident rayReflected ray — BN (Normal)PQ (mirror)O — point of incidence∠i = angle of incidence, ∠r = angle of reflection; by the law of reflection ∠i = ∠r

Reflection of light at a plane mirror, with all the terms marked.

  • Incident ray: AO
  • Normal: ON
  • Angle of incidence: ∠AON (∠i)
  • Angle of reflection: ∠BON (∠r)
  • Point of incidence: O
  • Reflected ray: OB
6Study the following incident. Swara and Yash could see their images clearly in still water. Yash then threw a stone into the water and the images became blurred.

(i) Is there a relation between the reflection of light and the blurring of the images?

Yes. Still water has a smooth surface. So a clear image is formed. When the stone is thrown, the surface becomes uneven. Light is reflected in different directions. So the image becomes blurred.

(ii) Which types of reflection of light can you notice from this?

  • In still water: Regular reflection.
  • In disturbed water: Irregular reflection.

(iii) Are the laws of reflection followed in these types of reflection?

Yes. The laws of reflection are followed in both types. At every point, the angle of incidence is equal to the angle of reflection.

7Solve the following examples.

(a) If the angle between the plane mirror and the incident ray is 40°, what are the angles of incidence and reflection?

∠i = 90° − 40° = 50°
∠r = ∠i = 50°

The angle of incidence and the angle of reflection are both 50°.

(b) If the angle between the mirror and the reflected ray is 23°, what is the angle of incidence of the incident ray?

∠r = 90° − 23° = 67°
∠i = ∠r = 67°

The angle of incidence is 67°.

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