Refractive Surgery...
"Refraction" refers
to the way the eye focuses light, the source of everything we see. There
are three basic elements that determine the eye's ability to focus
light:
The shape of the cornea,
the eye's transparent outer membrane
The power of the lens,
which lies behind the pupil and completes the focusing process
The length of the eyeball
Normal vision
In an eye with normal vision light rays are refracted in such a way that they meet at
a light-sensitive membrane called the retina, which is located at the
back of the eye. There, light images are changed into electrical
impulses and sent by the optic nerve to the brain's vision-processing
center.
Refractive errors occur when
light rays entering the eye do not focus properly on the retina.
Refractive errors are not diseases, but rather normal differences in
visual ability. There are three basic types of refractive errors: myopia
(nearsightedness), hyperopia (farsightedness), and astigmatism.
Myopia
In myopia, the most common type of
refractive error, the cornea is too curved, the lens is too powerful, or
the eyeball itself is
too
long. As a result, light rays are refracted too sharply and meet at a
point in front of the retina. For this reason, faraway objects appear
blurry.
Hyperopia
People with hyperopia, on the other
hand, experience just the opposite problem. Because the cornea is too
flat, the lens is too weak, or the
eyeball
is too short, light rays do not have enough space to come together
before reaching the retina. As a result, hyperopic people see distant
objects more clearly than nearby objects.
Astigmatism
Astigmatism, which often occurs
with either myopia or hyperopia, is an irregular curvature of the
cornea. It causes light rays to reach the
retina
in different locations and thus prevents the eye from focusing clearly
at any distance.
Presbyopia
A fourth type of refractive
error, presbyopia, occurs as the eye ages and the lens loses its ability
to change focusing for close objects. Hence the need for bifocals or
reading glasses.
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