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What is meant by power of accommodation of the eye?

The ciliary muscles alter the focal length of the lens depending on the distance of the object. This ability of the eye to alter its focal length depending on the distance of the object is called its power of accommodation.


A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?

A person with myopia or short sight can be corrected using a concave lens (divergent lens) of suitable focal length. This lens has a negative power. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.


What is the far point and the near point of the human eye with normal vision?

For a young adult with a normal vision, the near point is about 25 cm and the far point of the eye is at infinity. A normal eye can see objects clearly that are between 25 cm and infinity.


A student has difficulty in reading the black board while sitting in the last row. What could be the defect of the child? How can it be corrected?

The child is suffering from myopia. It can be corrected using a concave lens (Divergent lens).


The human eye can focus objects at different distances by adjusting the focal length of the eye lens.

This is due to accommodation.


The human eye forms the image of an object at its ___________.



The least distance of the distinct vision for a young adult with normal vision is about ___________.

25 cm.


The change in focal length of an eye lens is caused by the action of the ________.

ciliary muscles.


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?

Power of the lens is given by the formula:
P= dioptre

(i) For distant vision the focal length is given by
f = = -= -.18 m = -18 m.

(ii) For near vision the focal length is given by
f = = = 0.66 m = 66 cm.


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?

The nature of the lens used to correct myopic eye is a convex lens.
The power to be used is :
Far point is 80 cm
U = -8 and v= -80 cm
= = =
f = -80 cm
P = = - = -1.25 D.


Why is a normal eye not able see clearly the objects placed closer than 25 cm ?

The focal length of the eye lens cannot be decreased below a certain minimum limit. If we try to read a printed page by holding it very close to the eyes, we may see the image appearing blurred or feel strain in the eye. We must hold it at about 25 cm from the eyes, to see an object comfortably and distinctly.


Why do stars twinkle?

The twinkling of a star is due to the atmospheric refraction of star light. The starlight on entering the earth’s atmosphere undergoes refraction continuously before it reaches the earth. The atmospheric refraction occurs in a medium gradually changing refractive index. The star appears slightly higher than its actual position when viewed near the horizon. Further this apparent position of the star is not stationery, but keeps on changing slightly. Since the physical condition of the earth’s atmosphere is not stationery. Since the stars are very distinct the approximate pint-sized sources of light. As the path of ray 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.


Explain why planets do not twinkle?

The planets are much closer to the earth, 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.


Why does the sun appear reddish early in the morning?

Light from the sun near the horizon passes through thicker layers of air and larger distance in the earth’s atmosphere before reaching our eyes. However, light from the sun overhead would travel relatively shorter distance. At noon, the sun appears white as only a little of the blue and violet colours are scattered. Near the horizon, most of the blue light and shorter wavelengths are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelengths. This gives rise to the reddish appearance of the sun.


Why does the sky appear dark instead of blue to an astronaut?

The scattered blue light from the sky enters our eye. If the earth had no atmosphere there would not have been any scattering. Then , the sky would have looked dark. The sky appears dark to passengers flying at a very high altitudes, as scattering is not prominent at such heights.

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