A cataract is a clouding or darkening that develops in the normally clear lens of the eye. This prevents the lens from properly focusing light on the retina (the back of the eye) resulting in a loss of vision. It is not a film that grows over the surface of the eye.
No one is exactly sure what causes a cataract but it is known that chemical changes within the lens cause it to become cloudy. This is often thought of as part of the natural aging process, but it may also result from the use of certain medications, heredity, injury, disease or excessive exposure to ultraviolet or infrared light.
Cataracts are most often found in persons over age 55, but are also occasionally found in younger persons, even newborns. Cataracts usually develop in both eyes but often at different rates. Some cataracts develop slowly over a period of years and others form rapidly within a few months. Cataracts can be treated very successfully by surgical removal followed by artificial lens implantation.