(i) When sunlight enters the earth's atmosphere, the atoms and molecules of different gases in the atmosphere absorb this light and re-emit light in all directions. This process is known as scattering of light. When sunlight passes through the atmosphere having the molecules of air and other fine particles, whose size is smaller than the wavelength of visible light, these molecules and particles scatter the blue colour more strongly than the other colours of spectrum as the wavelength of blue light enters our eye. So, the colour of sky appears blue to us during day time.
(ii) The sun and the surrounding appear red at sunrise and at sunset. This is because, at the time of sunrise and sunset, the sun is near the horizon and so the sun has to travel greatest distance through the atmosphere to reach us. During this travel, most of the shorter wavelength present in it are scattered out from our sight and only the longer wavelength i.e., red colours is left out which makes the sun appears red.

(a) The ability of eye lens to adjust its focal length to form the sharp image of the object at varying distances on the retina is called its power of accommodation.
(b)To correct an object at infinity has to be brought as an image to 120 cm.
\(\therefore\ \ {1\over f}={1\over -120}-{1\over (-\infty)}={1\over -120}\)
\(\Rightarrow\ f=-120cm\)
\(P={100\over -120}={-5\over 6}D\)
\(=-0.83D\)
A concave lens of focal length 120 cm and power - 0.83 D is to be used.

(a) The ability of eye lens to adjust its focal length to form the sharp image of the object at varying distances on the retina is called its power of accommodation.
(b) When we see the nearby object the cillary muscles contract, it increases the thickness of eye lens. The eye lens then becomes thicker. As a result the focal length of eye lens decreases in such a way that the clear sharp image of nearby object is formed on the retina. Thus, the object is seen clearly to us. When we see the distant object these muscles becomes relaxed, thus the eye lens becomes thinner, and consequently focal length of the lens increases. Therefore, the parallel rays coming from the distant object are focused on the retina and object is seen clearly to us Thus the accommodation power of an eye helps a person to see nearby as well as distant objects clearly

Explanation :
The amount of light entering the eye can be controlled by the pupil. If the intensity of outside light is low, then the pupil expands to allow more light to enter the eye. On the other hand, if the outside intensity of light is high, then the pupil contractors so that less lights enters the eye.

Different colours travel through prism with different speeds and have different refractive index

It regulates the amount of light entering the eye by adjusting the size of the pupil.

The distance between the near point and far point is called range of vision.

The sky appears dark to the astronauts due to absence of atmosphere at very high altitudes.

A person who is blind to red-green colour may be deficient in cone shaped cell having red and green pigment in the retina of his eyes. It is a genetic disorder and not a refractive defect of vision. That is why such a person may have normal vision.

No, a person may have normal of accommodation and yet he may be myopic or hypermetropic. Infact. Myopia may arise when length of eye ball elongated and hypermetropia may arise when length of eye ball gets shortened.

Sky appears dark to the astronauts because there is no atmosphere and hence no scattering of light takes place in the space.

Stars twinkle due to atmospheric refraction of starlight. The starlight passes through different layers of atmospheric medium of gradually changing refractive index. So, it is refracted multiple times. As the apparent position of star and the physical condition of the earth's atmosphere are not constant the amount of starlight entering the eye flickers and gives a twinkling effect.

(a) (i) The iris and pupil control the amount of light entering the eyes. If the amount of light received by the eye is large, then the iris contracts the pupil and reduces the amount of light entering the eye.
(ii) If the amount of light received by the eye is small, the iris expands the pupil so that more light may enter the eyes.
(b) The screen on which the image is formed in the eye is called retina. Retina is a delicate membrane having a large number of light sensitive cells called 'rods' and 'cone' which respond to the 'intensity of light' and 'colors of object' respectively.
(c) Reasons for donating eyes:
(1) A person's corneal blindness can only be elevated by a human donor's cornea.
(2) It is opportunity to restore someone's sight.
Methods for supporting this cause:
(1) Make it a family tradition of donating the eyes.
(2) Motivate and educating others about eye donation.
(3) Help removing all the myths about eye donation.