1. The human eye can focus objects at different distances by adjusting the focal
length of the eye lens. This is due to
2. The human eye forms the image of an object at its
Ans:- (d) retina.
3. The least distance of distinct vision for a voung adult with normal vision Is about
(a) 25 m.
(d) 2.5 cm.
(c) 25 cm.
(b) 2.5 m.
Ans:- (c) 25 cm.
4. The change in focal length of an eye lens is caused by the action of the
(c) ciliary muscles.
Ans:- (c) ciliary muscles.
o. 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 dioptre. What is the focal
length of the lens required for correcting (1) distant vision, and (i) near vision?
O The lar 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?
J. Make a diagram to show how hypermetropia is corrected. The near point of a
hypermetropic 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.
8. Why is a normal eye not able to see clearly the objects placed closer than
Ans:- This is because the focal length of the eye lens cannot be decreased below a certain minimum limit.
9. What happens to the image distance in the eye when we increase the distance of an
object from the eye?
Ans:- For a normal eye, the image is always formed at the retina. As objects distance is increased the eye lens changes its focal length to keep image distance constant.
10. Why do stars twinkle?
Ans:- The twinkling of a star is due to the atmospheric refraction of starlight. The starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth. The atmospheric refraction occurs in a medium of gradually changing the refractive index.
11. Explain why the planets do not twinkle.
Ans.: The planets are much closer to the earth, and are thus seen as extended sources. If we consider a planet as a collection of large no of point sized sources of light, the variation in the amount of light entering our eye from all the individual point sized sources will average out to zero, thereby nullifying the twinkling effect.
12. Why does the Sun appear reddish early in the morning?
Ans:- At the time of sunrise, the sun is near the horizon. Light from the sun passes through thicker layers of air and a larger distance in the earth’s atmosphere before reaching our eyes. Therefore most of the blue light and shorter wavelengths are scattered away by the particles. The red colour which has the largest wavelength is scattered the fast and enters into our eyes. Hence the sun appears reddish.
13. Why does the sky appear dark instead of blue to an astronaut?
Ans:- The sky appears dark instead of blue to an astronaut, as a scattering of light does not take place outside the earth’s atmosphere.