The eye is an optical device designed to detect and focus light. It is a wonderfully complex and yet simple mechanism. The basic features of the eye are noted in the diagram below. A more complete picture can be found in any Biology textbook.

The eye is about the size of a ping pong ball. In the front is a hard transparent membrane called the cornea. It acts as a protective membrane for the eyeball. The pupil is the black circle in the center of the eye. The pupil is the “window” that allows light in. The iris is where the color of your eye is; that is, if you have blue eyes, it is your iris that is blue in color. The iris controls the amount of light that enters your eye. Under low-light conditions such as nighttime the iris will open up, causing the pupil to become larger and allowing more light in. During the day or in a bright room, the iris will close slightly, since the eye does not need as much light to see. (Some animals, such as cats, have irises that are shaped like vertical slits. At night they are wide open, causing their pupils to be round)

The lens is the device that bends the light rays. It is a convex lens, and produces an inverted real image on the retina.

The inverted image is sent via the optic nerve to the vision center of the brain, the visual cortex, which is located at the very back of the brain. The brain inverts the image it receives, so the person sees the world right-side-up. You can prove this to yourself with an easy activity.

Some people need corrected vision. The lenses in their eyes do not focus the image exactly at the location of the retina. Two common conditions are nearsightedness and farsightedness.

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