Near & Farsightedness, What are visual defects and how common are they?

Essay by Anonymous UserJunior High, 8th gradeA+, March 1997

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        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.

page 1

        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...