When light enters the eyes, it passes first through the cornea, a transparent cover, and then through a watery fluid called the aqueous humor, and through the pupil, which is an opening in the colored part of the eye called the iris. The size of the pupil increases when there is less light and decreases when there is more. Light next passes through the lens, which focuses the light through a transparent jell, called the vitreous body, onto the retina. The retina has two types of light sensitive cells, rods and cones. Rods are particularly sensitive to shades of light and outlines, and they are important in both night and peripheral vision.
Cones have specialized pigments that are sensitive to either red, green, or blue.
Cones enable people to see details. Rods and cones cover the entire retina except for a small area just above the optic nerve, known as the blind spot. When a person looks at an object of a particular color for a long time and then suddenly looks at a blank space, the person sees a ghostly outline of the object in its complimentary color. If the object is red, the person sees green. If the object is blue, the person sees yellow.