Exercise : Chapter 11
Q1. 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.
Ans: (b) accommodation.
Q2. The human eye forms the image of an object at its
(a) cornea. (b) iris (c) pupil. (d) retina.
Ans: (d) retina
Q3. 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) 25 cm
Q4. 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
Q5. 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 (i) distant vision, and (ii) near vision?
Ans:
Q6. 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?
Ans:
Q7. 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:
Q8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Ans: The minimum distance of distinct vision of a human is 25cm. Rays of light reflected off an object placed at a distance of less than 25 cm will not make the object visible on the retina. Because the capacity of the human eye cannot be increased by 25 cm.
Q9. What happens to the image distance in the eye when we increase the distance of an object from the eye?
Ans: Image distance always remains the same. The reason for this is that the distance of the object, the focal length of the lens of the human eye is adjusted in such a way that the image is formed on the retina.
Q10. Why do stars twinkle?
Ans: The refractive index of the Earth's atmosphere is constantly changing. The light of the stars entering the eyes remains irregular due to constant refraction and due to that flicker the stars appear to twinkle.
Q11. Explain why the planets do not twinkle.
Ans: Earth's distance from the planets is very less. Planets are stores of light. The light rays that come from the planets do not have refraction. Their position does not change with the proximity and the store of light, so they do not appear to twinkle.
Q12. Why does the Sun appear reddish early in the morning?
Ans: The sun is on the horizon at sunrise or sunset. In that case, the sun's rays first reach the thick layers of air in the earth's atmosphere, after that to our eyes. Most of the light of short wavelength is scattered by the particles of the atmosphere. In this way only long light rays (red) can enter our eyes and we see the sun as blood-stained.
Q13. Why does the sky appear dark instead of blue to an astronaut?
Ans: Due to the absence of an atmosphere in space, there is no scattering of light, because only the fine particles of the air scatter the light. This is the reason why the sky appears black to the astronauts.
