Light enters via the clear cornea of the eye. Its intensity is controlled by the adjustable diaphragm, the iris. The light passes through the iris opening called the pupil, and is focused by the lens on the retina. From the retina the light is converted into electrical impulses, conducted by the optic nerve and tract to the occipital cortex or the back part of the brain. For the brain to get the fullest information the image formed should be sharply focused and clear.
What are refractive or focusing errors of the eye? What is accommodation?
The light which enters the eye as parallel rays has to be bent to focus at a point. Refraction means the bending of light rays when they travel from one medium, say air, into another medium, say water, of different density.
The cornea or the clear membrane is the point where the first refraction occurs, for it is denser than air. There are other liquid media in the eye which also bend the light rays, but the most important is the lens. The lens acts as a variable continuous focusing system. It has the ability to become rounder, thus increasing the power or the focusing capacity. It can also become flatter and decrease the focusing capacity. The ability of the lens to change its focus is termed accommodation and is completely under the control of the brain. A blurry retinal image triggers off accommodational effort which clears the image and focuses it to exquisite sharpness.
Another important fact is that the two eyes accommodate or focus together. They cannot work independently.