(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.

(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.

(i) Since the sun is near the horizon early morning, it has to travel greatest distance through the atmosphere to reach us. During this travel, most of the shorter wavelengths present in it are scattered out from our sight and only the longer wavelength i.e., red colour is left out which makes the sun appears red.
(ii) This phenomenon will not be observed by an observed on the moon as there is no atmosphere in the moon to scatter to light.

(i) When a beam of white light is passed through a prism, a band of seven colors is formed on a white screen. This is called spectrum.
(ii) The dispersion of white light occurs because the angle of refraction or angle of refraction or angle of bending of different components of the white light is different when passing through the glass prism. When white light consisting of seven colours falls on a prism, each colour is refracted or deviated by a different angle with the result that the seven coloured band i.e., spectrum is formed.

(i) Twinkling effect. 
(ii) It is due to atmospheric refraction of starlight. The atmosphere has varying layers. The gradual change in the refractive index of different layers of atmosphere causes atmospheric refraction and starlight undergoes multiple refractions. So, the fluctuation in the position of star occurs continuously due to the changing amount of light entering the eye. The star sometimes appears brighter and at some times, it appears fainter. This cause twinkling of star.

(i) The clear sky appears blue during day time.
(ii) When sunlight passes through the atmosphere having the molecules of air and other fine particles, whose size is smaller than the other colors of spectrum as the wavelength of blue colour is more. This scattered blue light enters our eye. So, the  colour of sky appears blue to us during day time.

(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

During sunrise/sunset, the looks reddish. This is because at this because at this stage, rays from the sun have to travel a much larger part of atmosphere. The red colour having largest wavelength is scattered the least.
At noon, the sun is nearly overhead. The sunlight has to pass through much smaller portion of earth's atmosphere the scattering is much less and the sun looks white.

When sunlight passes through the atmosphere, most of the light having longer wavelengths do not get scattered much. But the light of shorter wavelength scattered all around the sky by air molecules present in the atmosphere. Some of this scattered blue light enters our eyes and so in whichever direction we look, the sky looks blue.
Thus, we can say that the sky appears blue due to the molecules in air which scatter the blue part of the sunlight much more easily than the red light or other shades.

After rain, a large number of tiny rain drops remain suspended in air, when sunlight falls on these droplets, they get refract and dispersed in the atmosphere thereby each raindrops act as a tiny glass prism splitting into a spectrum.
since each raindrops act like prisms, when white sunlight enters and leaves these raindrops, the coloured rays in white light refracted by different amounts and so an arch of seven colours i.e., rainbow is formed.

(i) The structures of the eye such as cornea, pupil, ciliary muscles, eye lens, aqueous humour and vitreous humour etc.,
(ii) The retina of the eye which converts light signals to electrical impulses and 
(iii) Optic nerve that transmit electrical impulses to brain.

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.