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1. The human eye can focus on objects at different distances by adjusting the focal length of the eye lens. This is due to
Ans: (b) Accommodation
2. The human eye forms the image of an object at its
(a) Cornea (b) Iris (c) Pupil (d) Retina
Ans: (d) Retina
3. The least 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
Ans: (c) 25cm
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
Ans: (c) ciliary muscles
7. 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.
- Ans: In hypermetrophia the focal length of the eye lens increases and power decreases, hence image of a near object is created after retina. So, for its correction, the focal length of the eye lens should be decreased and power should be increased.
- Hence a convex lens of suitable focal length or suitable power will bring the image back on to the retina and thus the defect is corrected by a suitable eye-glass having converging or convex lens.
- Again given that, the near point of a normal eye is 25 cm hence it is taken as “u”or object distance.
But the, as the hypermetropic eye has near point 1m, it is taken as image distance or “v”
8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
- Ans: the near point of the eye is 25cm. If an object placed less than 25 cm from the eye, the ciliary muscles fails to contract, hence the curved of eye lens can not decrease. Because the focal length of the eye lens decreases up to a certain distance.
- Hence, the image of the object can not be created at retina. Hence we can not able to see a object placed closer than 25 cm clearly.
9. What happens to the image distance in the eye when we increase the distance of an object from the eye?
Ans: the image distance in the eye does not change, when we increase the distance of an object from the eye. It happens, because of two causes;
- The image of any object is formed at retina of the eye irrespective of the object distance.
- Rather, when the distance of the object is changed, the eye lens increases or decreases its focal length by the help of ciliary muscles according to the situation.
10. Why do stars twinkle?
- Ans: The twinkling of a star is due to atmospheric
refraction of starlight. The starlight, on entering the earth’s atmosphere,
undergoes refraction continuously before it reaches our eye.
- Because the refractive index of air gradually increases when we move towards the ground. Hence the light from the
star i.e star light, gradually moves towards the normal. For which the apparent position of the star is slightly different from its actual position. The star appears slightly higher (above) than its actual position when viewed near the horizon.(see pic 11.10)
- Further, this apparent position of the star is not stationary, but keeps on changing slightly, since the physical conditions of the earth’s atmosphere are not stationary.
- Since the stars are very distant, they approximate point-sized sources of light. As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of starlight entering the eye flickers – the star sometimes appears brighter, and at some other time, fainter, which is the twinkling effect.
11. Explain why the planets do not twinkle.
Ans: The planets do not twinkle because,
- The planets are much closer to the earth than the stars, and are thus seen as extended sources.
- If we consider a planet as a collection of a large number of point-sized sources of light, the total 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: During the Sunrise the Sun is at the horizon. Hence the light rays, those come from the Sun travels through the different layers of the atmosphere and travel a longer path.
- Hence, most of the smaller wavelength blue lights are scattered by the tiny particles of the atmosphere.
- The light rays having a longer wavelength are scattered very low. Hence the longer wavelength red light falls on our eye. For which, the colour of the Sun looks bright Red during Sunrise
13. Why does the sky appear dark instead of blue to an astronaut?
Ans: The astronauts generally fly on very high altitudes. There is no atmosphere at very high altitudes. Hence, the scattering of light is not prominent at such heights. Hence the sky appears dark instead of blue to the astronaut.