What Are Visual Defects and How Common Are They ?


Nate Schackow 2nd Period December 17, 1996

The human eye does alot more than allowing you to see. It is very
complex and has many parts and features which can have defects. However, to
understand defects you must first know how the eye works.
First light passes through the cornea, which is the transparent part of
the sclera, or white of the eye, which is composed of tough fiberous tissue.
Behind the sclera is a watery fluid called the aqueous humor. This fluid fills
a cresent-shaped space which with the cornea helps bend the light toward the
center of the eye.
Under the aqueous humor is the iris which gives the eye color. The
color of the iris has no effect on how you see and is inherited through genes.
The iris contols how much light is allowed to enter your by opening up further
when it is dark and closing up more to block out some light when it is bright.
Everything that passes through the pupil, which looks like a black dot, is what
you see.
Next the light passes through the lens. The lens focuses the light rays
onto the retina forming an image in reverse and upside-down. Finally light-
sensitive cells in the retina transmit the image via the optic nerve to the
brain by electrical signals. Then the brain flips the image so it looks right-
side-up to you. You can find a diagram of the above on page 3.
The most common visual defects are nearsightedness and farsightedness.
In nearsightedness, also known as myopia, the eye is longer than usual. This is
corrected by using a concave lens to spread the light rays just enough to
increase the eye's focal length. Hyperopia, also known as farsightedness, is
caused by a shorter than usual eye. A convex lens increases light bending and
returns the point of focus to the retina.


Coon, Dennis, Introduction to Psychology, St. Paul,
Minnesota, West Publishing Company, 1989, pp.
85-87. "Eye," Compton's Interactive Encyclopedia, 1994, 1995
Compton's NewMedia, Inc. Pierenne, M. H., Vision and the Eye, London,
Chapman and Hall Ltd., 1967, pp. 2-9.

Category: Science