The eye is one of the most complex and remarkable structures in the human body. In some ways it is the most sensitive of all our sense organs and can perceive things, such as distant galaxies, that none of our other senses can. The eye is rather like a sphere with a bulge at the front. It has a tough, white outer layer, called the sclera, and where the sclera rounds off the bulge at the front of the eye, it forms the cornea, which is clear. The conjunctiva is a very thin protective layer and is fairly loosely connected to the eyeball, except at the point where the cornea and the sclera meet; the conjunctiva covers the sclera, but not the cornea.

In order to function, the eye needs light. Light, usually reflected by objects, enters the front of the eye through the cornea. Behind the cornea is the lens. It is the cornea which does most of the focusing, but the lens which fine-tunes the focusing and enables the eye to vary its focus. An image is formed by the cornea and lens on the retina which receives light and changes it into electrical signals which are then transmitted to the brain by the optic nerve.

Between the cornea and the lens, light passes through the pupil. (the black spot in the middle of the eye). The iris, which is the colored part of your eye, is actually a ring of very fine muscles which make the pupil larger or smaller, depending on how much light the eye needs to make an image. In bright conditions, the pupil shrinks (constricts) to let in less light, while in darker conditions it lets in more light by getting bigger (dilates). The eye is comparable to a camera in which the lens focuses the picture on a light sensitive film at the back.