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(a) What happens to the size of pupil of our eye (i) in dim light (ii) in bright light?(b) Name the cells on the retina of an eye which are sensitive to (i) bright light (ii) dim light (iii) sensation of colour.
(a) The size of the pupil of our eye
(i) in dim light $-$ it becomes large so as to accommodate more light in the eyes.
(ii) in bright light $-$ it becomes small so that very little light can enter the eye.
Explanation
Pupil is a hole in the middle of the iris, which appears black because the light that passes through it, gets absorbed by the retina and is not reflected back. Its function is to control the amount of light entering the eye with the help of the iris so that it can be focused on the retina to begin the process of sight.
In low-light conditions, the pupil expands or dilates, so that more light can reach the retina to improve night vision.
In bright conditions, the pupil shrinks or constricts to limit light entering into the eyes, so that too much light can't cause glare and discomfort to the eye, and even can't damage to the lens and retina of the eye.
(b) The cells on the retina of an eye which are sensitive to
(i) bright light $-$ cones (cone-shaped cells).
(ii) dim light $-$ rods (rod-shaped cells).
(iii) sensation of colour $-$ cones.
Explanation
The retina of our eye contains a large number of light-sensitive cells that respond to light. These specialized cells are known as photoreceptors.
There are two different types of photoreceptors present within the retina:
1. Rods - They are most sensitive to light and dark changes, hence function better in low light conditions (twilight vision) and get bleached quickly in bright light. They are not good for color vision. There are about 120 million rods found in the human retina.
2. Cones - They are not as sensitive as rods for light. Therefore, they function better in bright light conditions (daylight vision), and require more photons of light to become activated. It also makes us see colors, along with helps us to differentiate between various colors. That's why we are unable to see color very well in dark places. Thus, they are used for color vision and for detecting fine details. There are about 6 million cones found in the human retina.
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