1. (b) accommodation
2. (d) retina
3. (c) 25 cm
4. (c) Cilliary muscles
5. (i) Power of lens needed for correction distant vision of the person (P) = -5.5 D
Focal length of lens required for correcting distant vision (f)
= 1/P = 1/-5.5 m = 0.18m = 18 cm.
(ii) For correcting near vision the power of lens required (P) = +1.5 D
Focal length of lens required for correcting near vision (f)
= 1/P = 1/1.5 m = 0.67 m = 66.7 cm.
6. To correct the myopia the person concerned should use concave lens of focal length
(f) = -80 cm = -0.80 m
Power of lens (P) = 1/f(m) = 1/-0.80 = 100/-80 = -1.25 D.
7. Diagram representing the correction of hypermetropia is a follows:
Near point of defective eye is 1 m and that of normal eye is 25 cm.
Here, u = -25 cm, v = -1m = 100 cm.
Using lens formula
1/f = 1/v – 1/u
1/f = 1/-100 + 1/25 = 3/100
f = 100/3 cm = 1/3m.
P = 1/f(m) = 1/0.33 = +3.0 D.
8. Due to limit of power of accommodation, the focal length of the eye lens cannot be
decreased below certain minimum limit. So, a normal eye cannot see clearly the
objects placed closer than minimum distance, called near point of the eye.
9. The image is formed on the retina even on increasing the distance of an object from
the eye. In fact, the eye lens becomes thinner and its focal length increases as the
object is moved away from the eye and consequently image is formed on the retina.
10. Stars twinkle due to atmospheric refraction of starlight. As the stars are very away
they behave as almost point sources of light. As on account of atmospheric
refraction, the path of rays of light coming from the star goes on varying slightly, the Material downloaded from http://myCBSEguide.com and http://onlineteachers.co.in
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apparent position of the star fluctuates and the amount of light entering the eye
flickers, so sometimes the star appear brighter and at some other time, fainter. Thus
the stars twinkle.
11. Planets are much closer to the earth and are seen as extended source. So, a planet
may be considered as a collection of a large number of point-sized light sources.
Although light coming from individual point-sized sources flickers but the total
amount of light entering our eye from all the individual point-sized sources average
out to be constant. Thereby, planets appear equally brighter and there is no
twinkling of planets.
12. In early morning, the Sun is situated near horizon. Light from the Sun passes through
thicker layers of air and covers a larger distance in earth’s atmosphere before
reaching our eyes. While passing through atmosphere blue light is mostly scattered
away and the Sun appear reddish.
13. Blue colour of the sky is on account of scattering of light of shorter wavelength by
particles in the atmosphere of earth. If the earth had no atmosphere, there would not
have been any scattering and sky would have looked dark. When astronaut in his
spacecraft goes above the atmosphere of earth, sky appears dark to him because