### Intext Questions Solved

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Question 1 : What is meant by power of accommodation of the eye?

Answer : The ability of the eye lens to adjust its focal length is called accommodation.

Question 2 : A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of corrective lens used to restore proper vision?

Answer : The lens should be concave (diverging).

Focal length of concave lens,

f = -(for point of defective eye)

f = -1.2 m

M = -120 cm

Question 3 : What is the far point and near point of the human eye with normal vision?

Answer : For normal eye : Near point is at 25 cm and far point is at infinity.

Question 4 : A student has difficulty to reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?

Answer : He is suffering from myopia or short sightedness. It can be corrected by using a concave lens of suitable focal length.

### Text Book Questions Solved

Question 1 : The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This is due to

a) presbyopia
b) accommodation
c) near-sightedness
d) far-sightedness

Question 2 : The human eye forms the image of an object at its

a) cornea
b) iris
c) pupil
d) retina

Question 3 : The lest distance of distinct vision for a young adult with normal vision is about

a) 25 m
b) 2.5 cm
c) 25 cm
d) 2.5 m

Question 4 : The change in focal length of an eye lens is caused by the action of the

a) pupil
b) retina
c) ciliary muscles
d) iris

Question 5 : A person needs a lens of power = -5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptres. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?

Answer : i) Given power of concave lens = -5.5D.

Question 6 : The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?

A concave lens of power -1.25 D will enable him to see far-off objects distinctly.

Question 7 : Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropia eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.

Question 8 : Why is a normal eye not able to see clearly the objects placed closer than 25 cm?

Answer : Because ciliary muscles cannot contract the eye lens beyond a limit that is why a normal eye is not able to see objects closer him 25 cm.

Question 9 : What happens to the image distance in the eye when we increase the distance of an object from the eye?

Answer : Image distance in the eye remains the same as object distance increases. This is because focal length of eye lens keeps changing.

Question 10 : Why do stars twinkle?

Answer : You must have noticed twinkling of stars on a clear night. When light travels from one medium to another, it undergoes refraction, due to which its path changes. In our atmosphere there are various layers of air which are having different densities, due to which their reactive indices are also different, i.e. as we came towards the surface of the carts from the atmosphere refractive index increases. When a ray of light enters the atmosphere, it undergoes multiple refractions, due to which apparent position of star keeps on changing as shown in figure below as a result, intensity of light coming from stars also fluctuate with time and a star appears to be twinkling.

Question 11 : Explain why the planets do not twinkle.

Answer : The planets are much closer to the earth compared to the stars. A planet can be considered as a large number of point sources and the variation in the amount of light emerging our eye from these point sources is zero therefore, twinkling effect does not occur.

Question 12 : Why does the sun appear reddish early in the morning?

Answer : During morning the sun is near horizon and the light from the sun passes through thick layers of air and larger distance in the earth’s atmosphere due to which blue colour gets scattered away, only red and yellow colour reaches our eyes.

Question 13 : Why does the sky appear dark instead of blue to an astronaut?

Answer : As there is no atmosphere there is no scattering of light. As a result the sky appear dark to an astronaut.