Human eyes work much like a camera. They allow images of a person's surroundings to be perceived by the brain and permit him or her to perceive different shapes, sizes, and colors. The eye is the classic subject of study of geometric optics, an important subclassification of modern physics. From studies of the eye, optical physicists have derived not only modern cameras and telescopes, but lenses to correct abnormal vision. They have also used the science of optics to understand the nature and motion of light.

The eye sits in a small, hollow area in the skull called the eye socket. Eyelids protect the eye from excessive light and foreign objects. The iris is the central, colored, part of the eye. It has muscles that control how much light enters the pupil, that black circular hole in the center of the iris. The pupils...