The sky appears dark instead of blue to an astronaut because there is no atmosphere in the outer space that can scatter the sunlight. As the sunlight is not scattered, no scattered light reach the eyes of the astronauts and the sky appears black to them.

1. A person cannot see distinctly objects kept beyond 2 m. This defect can be
corrected by using a lens of power
(a) + 0.5 D
(b) – 0.5 D
(c) + 0.2 D
(d) – 0.2 D
Ans. (b) -0.5 D
Explanation: This person is suffering from myopia. He needs a concave lens and hence power would be in negative.
P = 1 /f = 1/2m =0.5D

2. A student sitting on the last bench can read the letters written on the blackboard but is not able to read the letters written in his text book. Which of the following statements is correct?
(a) The near point of his eyes has receded away
(b) The near point of his eyes has come closer to him
(c) The far point of his eyes has come closer to him
(d) The far point of his eyes has receded away
Ans. (a) The near point of his eyes has receded away
Explanation: In hypermetropia the near point of eye moves away from 25cm. Due to this, the person needs to keep a book at more than 25 cm to read it properly.

3. A prism ABC (with BC as base) is placed in different orientations. A narrow beam of white light is incident on the prism as shown in Figure 11.1. In which of the following cases, after dispersion, the third colour from the top corresponds to the colour of the sky?

(a) (i)
(b) (ii)
(c) (iii)
(d) (iv)
Ans. (b) (ii)
Explanation: If prism is kept with base BC at bottom, then the emergent band of colour would show violet at the bottom. If prism is kept with base BC at top, then violet would be at top; followed by indigo and blue.

4. At noon the sun appears white as
(a) light is least scattered
(b) all the colours of the white light are scattered away
(c) blue colour is scattered the most
(d) red colour is scattered the most
Ans. (b) all the colours of the white light are scattered away
Explanation: Sky will appear dark in case of option ‘a’. It will appear blue in case of option ‘c’ and will appear red in case of option ‘d’.

5. Which of the following phenomena of light are involved in the formation of a rainbow?
(a) Reflection, refraction and dispersion
(b) Refraction, dispersion and total internal reflection
(c) Refraction, dispersion and internal reflection
(d) Dispersion, scattering and total internal reflection
Ans. (c) Refraction, dispersion and internal reflection
Explanation: Dispersion results in white light getting segregated into its component colours. Refraction bends the incident light to an angle that is causes internal reflection; and finally rainbow is formed.

6. Twinkling of stars is due to atmospheric
(a) dispersion of light by water droplets
(b) refraction of light by different layers of varying refractive indices
(c) scattering of light by dust particles
(d) internal reflection of light by clouds
Ans. (b) refraction of light by different layers of varying refractive indices
Explanation: Due to refraction of light by different layers of varying refractive indices, the apparent position of source of light keeps on changing. Due to this, stars appear to twinkle.

7. The clear sky appears blue because
(a) blue light gets absorbed in the atmosphere
(b) ultraviolet radiations are absorbed in the atmosphere
(c) violet and blue lights get scattered more than lights of all other colours by the atmosphere
(d) light of all other colours is scattered more than the violet an blue colour lights by the atmosphere
Ans. (c) violet and blue light get scattered more than lights of all other colours by the atmosphere

8. Which of the following statements is correct regarding the propagation of light of different colours of white light in air?
(a) Red light moves fastest
(b) Blue light moves faster than green light
(c) All the colours of the white light move with the same speed
(d) Yellow light moves with the mean speed as that of the red and the violet light
Ans. (c) All the colours of the white light move with the same speed

9. The danger signals installed at the top of tall buildings are red in colour. These can be easily seen from a distance because among all other colours, the red light
(a) is scattered the most by smoke or fog
(b) is scattered the least by smoke or fog
(c) is absorbed the most by smoke or fog
(d) moves fastest in air
Ans. (b) is scattered the least by smoke or fog

10. Which of the following phenomena contributes significantly to the reddish appearance of the sun at sunrise or sunset?
(a) Dispersion of light
(b) Scattering of light
(c) Total internal reflection of light
(d) Reflection of light from the earth
Ans. (b) Scattering of light
Explanation: Red colour scatters the least and hence travels the farthest. During sunset or sunrise, light has to travel a longer distance to reach us. Hence, only red light reaches to us and the sky appears reddish.


11. The bluish colour of water in deep sea is due to
(a) the presence of algae and other plants found in water
(b) reflection of sky in water
(c) scattering of light
(d) absorption of light by the sea
Ans. (b) Reflection of sky in water
Explanation: Water is colourless. Its colour appears to be same as the object reflected by it.

12. When light rays enter the eye, most of the refraction occurs at the
(a) crystalline lens
(b) outer surface of the cornea
(c) iris
(d) pupil
Ans. (b) Outer surface of the cornea

13. The focal length of the eye lens increases when eye muscles
(a) are relaxed and lens becomes thinner
(b) contract and lens becomes thicker
(c) are relaxed and lens becomes thicker
(d) contract and lens becomes thinner
Ans. (a) are relaxes and lens becomes thinner

14. Which of the following statement is correct?
(a) A person with myopia can see distant objects clearly
(b) A person with hypermetropia can see nearby objects clearly
(c) A person with myopia can see nearby objects clearly
(d) A person with hypermetropia cannot see distant objects clearly
Ans. (c) A person with myopia can see nearby objects clearly
Explanation: This is the reason; myopia is also known as near sightedness.

This student is unable to see far off objects. This means that the student is suffering from myopia. Doctor will prescribe a concave lens of suitable focal length.

Human eyes have power of accommodation. When we have to see distant objects, the eye muscles relax and lens becomes thin. Due to this, the focal length of the lens increases and the eye is able to see distant objects. When we have to see nearby objects, the eye muscles contract and lens becomes thick. Due to this, the focal length of the lens decreases and the eye is able to see nearby objects.

A person needs a lens of power –4.5 D for correction of her vision. (a) What kind of defect in vision is she suffering from? 

Ans. Myopia
(b) What is the focal length of the corrective lens?

P= 1 / f  or f= 1/ P

= -0.22 D

(c) What is the nature of the corrective lens?

Ans. The negative sign shows that it is a concave lens.

For this, one prism is placed near another prism so that one prism is in erect position and another prism is in inverted position. When ray of white light enters the first prism, dispersion of light takes place. When lights of different colours pass through the second prism, they recombine to make a ray of white light