How does the human eye see?

The eye is really an extension of the brain. It consists of a lens to focus the image, an iris to regulate the amount of light entering the eye, and a screen called the retina. The cells of the retina do some preliminary processing of the information they receive then send signals along the optic nerve to the brain.

The cornea is a transparent membrane on the outer surface of the eye. Between the cornea and lens is a fluid. Light refracts when going through the convex surface of the cornea into the fluid. In fact most of the focusing of light in the eye occurs at the cornea. Light passes through the iris that opens and closes in response to the amount of light entering the eye. The iris can only change the amount of light to a going through it by a factor of twenty, while our eye can respond to differences in light level of ten trillion! The major task of the iris then can’t be to control light intensity. In addition, when the opening in the iris shrinks the eye can keep objects in focus from a wider range of distances. After passing through the iris the light goes through the lens.

The lens consists of layers of transparent fibers covered by a clear membrane. In order to focus in on objects that we want to see, our eye changes the shape, and thus the focal length of the lens and cornea by contracting or relaxing the ciliary muscle around the eye. Light then passes through a liquid called the vitreous humor that fills the major volume of the eye and falls on the retina. The cornea and lens have created an inverted image on the surface of the retina.

The retina is composed of a layer of light-sensitive cells, a matrix of nerve cells, and a dark backing. There are two kinds of light-sensitive cells: cones and rods. The 7 million cones are sensitive to high light levels and are concentrated around the fovea, the part of the retina directly behind the lens. Surrounding it are some 120 million rods that are sensitive to low light levels. The entire retina covers about five square centimeters. There aren’t 127 million nerves in the optic nerve that goes to the brain, so a system of nerves in the retina do some preliminary processing of the electrical signals produced by the rods and cones before sending the results to the brain.


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