Question

(a) Draw a simple diagram of the human eye and label clearly the cornea, iris, pupil, ciliary muscles, eye-lens, retina, optic nerve and blind spot.
(b) Describe the working of the human eye with the help of the above diagram.
(c) How does the eye adjust itself to deal with light of varying intensity?

Answer 1

(a)

(b) Working of the human eye:
The light from an object enters the eye through the cornea, a transparent substance. Behind the cornea, there is the iris, a ring-shaped membrane that has a hole in the middle called the pupil. The light passes through the pupil and falls on the eye lens, which is a convex lens made of a transparent, jelly-type material. Suspensory ligaments are attached to the eye lens on one end and to the ciliary muscles on the other end. The eye lens converges the light rays to form a real and inverted image of the object on the retina with the help of light-sensitive cells and generates electric signals. The signals are conveyed to the brain through the optic nerve, which then gives rise to the sensation of vision. There are no visual receptors at the junction of the optic nerve and the retina. This point is called the blind spot. The space between the eye lens and the retina is filled with a transparent, jelly-like substance called vitreous humour, which supports the back of the eye.

(c) The iris regulates the amount of light entering the human eye by automatically adjusting the size of the pupil according to the intensity of light that the eye receives. If the intensity of light is high, the iris contracts the pupil so as to reduce the amount of light entering the eye. If the intensity of light is low, the pupil expands to allow more light to enter the eye.