Eyes do not actually see objects -- what they see is the light that objects reflect. When these light rays enter the eye, they are absorbed and converted into electrical signals by retinal nerves. These signals are subsequently sent to the brain where they are interpreted as visual images.

In a normal eye, light rays enter the eye through the "cornea" (window of the eye) and are focused with the help of a "crystalline lens" behind the pupil at a point directly on the retina (the light sensitive nervous tissue at the back of the eyeball). However, only about four in ten people have normal visual acuity. For the rest, clear vision may be achieved by refocusing light rays with the use of corrective lenses.

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