# Notes Science class 7 Unit 9 Dispersion of Light

### School-Based Assessment 2022

1.     What happens, when light travels from a glass into the air at an angle?

Ans: When light passes from a glass into the air, it bends away from the normal. The angle of incidence is greater than the angle of refraction.

### 2.     What is the refractive index?

Ans: Refractive index is the ratio of the speed of light in a vacuum to its speed in the medium.

Refractive index of the medium = speed of light in a vacuum/speed in the medium.

3.     How can you calculate the refractive index of water?

Ans:

Refractive index of the water  =  speed of light in vacuum/speed ÷ in the water

4.     What happens when the primary colours of light are mixed in equal proportions?

Ans: When we mix the primary colours in equal proportion we get white light.

5.     Why do we see the colours of different objects?

Ans: We see the colours of different objects because the colour of an object is the colour of light it reflects.

### 6.    Define critical angle.

Ans: The angle of incidence for which the angle of refraction is 90  is called the critical angle.

### 7.     State the laws of refraction.

Ans: There are two laws of refraction.

1.              The incident ray, the refracted ray and the normal at the point of incidence, all lie in the same plane.
2.             The ratio of the speed of light in a vacuum to its speed in another medium is always constant.

8.     Why a pencil in a glass of water looks broken at the waterline?

Ans: A pencil in a glass of water looks broken at the waterline because of the refraction of light.

9.     Define angle of incidence.

Ans: The angle between the normal and the incident ray is called the angle of incidence. It is denoted by “ ’’.

10.            What is the angle of refraction?

Ans: The angle between the normal and the refracted ray is called the angle of refraction. It is denoted by “r”.

### 11.            Define critical angle.

Ans: The angle of incidence for which the angle of refraction is 90 is called the critical angle.

12.            Why do the clear swimming pools look shallower than their actual depth?

Ans: The clear swimming pools look shallower than their actual depth because of the refraction of light.

13.            Define the spectrum of light.

Ans: Bypassing through the prism white light split into seven colours. The band of seven colours is called the spectrum of light.

## 14.            What is a dispersion of light?

Ans: The splitting of white light into its component colours is called dispersion of light.

15.            Explain the rainbow.

Ans: A rainbow is a natural demonstration of refraction, dispersion and total internal reflection of light.

16.            Which colours are present in a rainbow?

Ans: Rainbow has red, orange, yellow, green, blue, indigo and violet colours.

17.             Which colours are called primary colours?

Ans: Red, blue and green colours are used to make any other colour. These colours are called primary colours.

18.            How secondary colours are produced?

Ans: Secondary colours are produced by mixing any two primary colours.

1.     Define the refraction of light. Discuss the effects of refraction with examples.

### Refraction of light:

When light passes from one transparent medium to another, it changes its speed and direction (or bends). This bending of light is called refraction.

### Effects of Refraction:

1.              Whenever we open our eyes, we observe the refraction of light. The lens in our eye refracts light to form an image on the retina of our eye.
2.              A pencil in a glass of water as shown in the figure looks as if it has been broken at the waterline. It is because of the refraction of light.
3.            Refraction causes the formation of the rainbow.
4.            Refraction takes place in lenses used in spectacles, telescopes, and magnifying glasses.

2.     Define total internal reflection. Explain the phenomenon of mirage.

### Ans: Total internal reflection

When light passes from glass or water to air (denser to rare medium), it bends away from the normal. But when the angle of incidence (i ) is greater than the critical angle “c”, the light rays reflect in the same denser medium. This phenomenon is called total internal reflection. Total internal reflection takes place only when:

1.     Light passes from a denser medium (water or glass) to a rare medium (air).

2.     The angle of incidence of all rays must be greater than the critical angle of that denser medium.

## Mirage:

A mirage is an image of some distant object which appears to us due to the refraction and total internal reflection of light.

The air higher up is cooler than the air near the road. Light travels faster when it reaches the warmer air. The light ray bends as they travel downward due to refraction. Near the ground where the air is even warmer, the light rays travel almost parallel to the ground but continue to bend in other directions (total internal reflection). When we see these bending light rays, our brain assumes that the rays have travelled in a straight line. These rays seem to us as reflecting from the water. As a result, we see a mirage. Desert travellers often observe mirages.

#### 3.     What is a dispersion of light? Why does white light get dispersed?

Ans: The splitting of white light into its component colours is called dispersion of light. When a beam of light enters a prism, all the colours of white light refract at different angles; it causes the white light to split into its component colours. Red light bends the least. Violet light bends the most and refracts by the largest angle. In this way, white light disperses into its component colours.