Give an account of the structure of human eye.

Explanation

Structure of human eye:
i. Human eye is nearly spherical in shape with a diameter of about 2.4 cm.
ii. Light enters our eye through the thin membrane known as cornea. The maximum refraction of light rays entering the eye takes place from cornea,
iii. Behind the cornea there is a dark fleshy screen called iris. The colour of the iris is different for different people.
iv. At the centre of iris, there is a small opening of variable diameter, called pupil. The pupil controls and regulates the amount of light entering the eye.
v. For bright light, the pupil contracts, while in case of insufficient light, the pupil widens.
vi. There is a transparent biconvex crystalline lens located just behind the pupil also called as eye lens. This lens provides fine adjustment of focal length. For a normal eye in relaxed position, focal length of eye lens is about 2 cm.
vii. By adjusting focal length of eye lens, real and inverted image of an object is formed on the retina.

viii. Retina is a delicate membrane consisting of large number of light sensitive cells. When light falls on the retina, the cells get activated and produce electrical signals.
ix. These signals are sent to brain through optic nerve.
x. Ciliary muscles surround the lens. These muscles hold the eye lens in place. When the muscles relax, they increase the focal length of the lens, allowing the eye to see the objects that are far away. To see closer objects clearly ciliary muscles must contract in order to decrease the focal length of the lens.
xi. For nonnal human eye, the minimum distance of distinct vision is about 25 cm.