The eye functions much like a camera. Light rays enter the eye, passing through the cornea, the aqueous humor — transparent fluid in the front of the eye — and then the pupil and into the lens. The lens bends the light rays to focus objects onto the retina lining the back of the eye. From there, the image passes through the retinal cells, into the optic nerve, and finally to the back of the brain which processes the images.
Cataracts occur when there is a buildup of protein in the lens that makes it cloudy. This prevents light from passing clearly through the lens, causing some loss of vision. Since new lens cells form on the outside of the lens, all the older cells are compacted into the center of the lens resulting in the cataract.