How the Eyes See

The eye works much like a camera. It has "lenses" that focus light, and a "film" that records the focussed image. In the human eye, the principal 'lens' is the cornea, the curved, transparent layer of tissue in front of the iris. It is responsible for about 2/3 of the eye's focussing power. Behind the pupil is the lens of the eye, which aids in focussing light. The 'film' is the retina, the light-sensitive tissue that lines the back of the eye. In order to see clearly, the image must be brought into focus directly on the retina. The curvature of the cornea determines it's focussing power. In the 'normal' eye, the curvature of the cornea is an exact match for the length of the eye. This allows light to be focussed directly onto the retina, resulting in clear vision without the aid of glasses or contact lenses. The cells in the retina then convert the focussed light into electrical impulses which are transmitted, via the optic nerve, to the visual center in the back of the brain, thus resulting in 'vision'.