Question

(a) Explain the functions of the following parts of the eye:
(i) cornea
(ii) iris
(iii) pupil
(iv) ciliary muscles
(v) eye-lens
(b) If you walk from a dark room into sunlight and back again into dark room, how would your pupils alter in size? What makes this happen?
(c) Explain why, we cannot see our seats first when we enter a darkened cinema hall from bright light but gradually they become visible.

Answer 1

(a) Functions of the parts of the eye:
(i) Cornea: The front part of the eye that is made of a transparent substance and is convex in shape is called the cornea. The light from an object enters the eye through the cornea.

(ii) Iris: The iris is a flat, coloured and a ring-shaped membrane with a hole in the middle called the pupil. The iris regulates and controls the amount of light entering the eye by automatically adjusting the size of the pupil according to the intensity of light that the eye receives.

(iii) Pupil: The pupil is a hole in the middle of the iris and appears black because there is no light reflected from it. The pupil expands or contracts to regulate the amount of light entering the eye.

(iv) Ciliary muscles: The ciliary muscles contract or relax the eye lens and help in changing its focal length to focus the images of nearby or distant objects on the retina.

(v) Eye lens: The eye lens is a convex lens whose main function is to the converge the light rays from an object and form a real and inverted image of the object on the retina.

(b) Initially, in a dark room, the pupil is large so as to allow a large amount of light into the eye. When we go out into sunlight, we feel glare in our eyes as the pupil is large. Gradually, the pupil contracts, and we are able to see clearly. On moving back from sunlight to a dark room again, we are not able to see the surroundings clearly, because the pupil is small. Gradually, the pupil expands, which then improves our vision. This action of the pupil is controlled by the iris, which automatically adjusts the size of the pupil according to the intensity of light that the eye receives.

(c) When we enter a darkened cinema hall from bright light, the pupil is small. As very little light enters the eye, we cannot see our seats clearly. Gradually, the size of the pupil expands, allowing more light to enter the eye, which then enables us to find our seats.