Exercise Solutions | NCERT
1. The human eye can focus on 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
2. The human eye forms the image of an object at its
(a) cornea
(b) iris
(c) pupil
(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
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
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 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?
(i) Focal length f is given by,
f = 1/P
or
f = 1 / -5.5
= -0.1818 m
= -18.18 cm
(ii) Focal length f is given by,
f = 1/P
or
f = 1 / +1.5
= +0.6667 m
= +66.67 cm
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?
Ans. For myopia, focal length is given by,
f = -x = -80 cm = -0.8 m
Power, P = 1/f = 1 / -0.8
= -1.25 dioptres.
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.
Answer:
Here
u = - 25 cm
v = - 1 m = -100 cm
\frac{1}{f}=\frac{1}{v}−\frac{1}{u}
\frac{1}{f}=\frac{1}{-100}−\frac{1}{-25}
\frac{1}{f}=−\frac{1}{100}+\frac{1}{25}
\frac{1}{f}=\frac{−1+4}{100}
\frac{1}{f}=\frac{3}{100}
f=\frac{100}{3}cm
or f=\frac{1}{3}\;m
Power,\;P=\frac{1}{f}
P=\frac{1}{\frac{1}{3}}
P=+3\;D
Diagram
8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Answer : A normal eye is unable to clearly see the objects placed closer than 25 cm because the ciliary muscles of eyes are unable to contract beyond a certain limit. If the object is placed at a distance less than 25 cm from the eye, then the object appears blurred and produces strain in the eyes.
9. What happens to the image distance in the eye when we increase the distance of an object from the eye?
Answer : The size of eye cannot be increased or decreased. Therefore, the image distance remains constant. When we increase the distance of an object from the eye, the image distance in the eye does not change. The increase in the object distance is compensated by the change (increase) in the focal length of the eye lens such that the image is formed at the retina of the eye.
10. Why do stars twinkle?
Answer : Twinkling of stars can be seen on a clear night. This is due to atmospheric refraction of light coming from the stars (star light). As the star light enters into the earth’s atmosphere, atmospheric refraction takes place due to gradually changing refractive index of the air. Since the physical conditions of the refracting medium (earth’s atmospheric) are not stationary, star light flux (luminous flux) entering the eye of an observer continuously fluctuates. This means luminous energy entering the eyes per second from the star increases and decreases with time. Thus, the star sometimes appear brighter and at some other time fainter, causing the ‘twinkling of stars’.
11. Explain why the planets do not twinkle.
Answer : The apparent size of stars is very small as compared to apparent size of planets. Thus, the star may be considered as a ‘point sized’ source of light and the planet as an ‘extended source’ of light. So, the planet can be considered as a collection of large number of ‘point sized’ sources of light, such that the dimming effect of some point sources’ is nullified by the brighter effect of the other ‘point sources’. The variable atmospheric conditions are unable to create variations in light flux from the planet entering our eye and thus, planets do not twinkle.
12. Why does the sky appear dark instead of blue to an astronaut?
Answer : The sky appears dark instead of blue to an astronaut because there is no atmosphere in the outer space that can scatter the sunlight. As the sunlight is not scattered, no scattered light reaches the eyes of the astronauts and the sky appears black to them.