Refraction is the passage of light through a medium
of varying density. The following section offers a simple
overview of how light passing through the eye affects
the quality of vision
The Normal Eye
Light rays are reflected off objects. When we look
at an object the light rays enter the eye and fall on
the back of the eye, or the retina. The retina then
translates the light rays into electrical impulses that
the brain interprets as visual images. A refractive
error occurs when the rays of light that enter the eye
do not fall correctly on the retina resulting in blurred
vision. There are three types of refractive error; nearsightedness
(myopia), farsightedness (hyperopia), and astigmatism.
When light enters the eye it first passes through the
cornea, the transparent covering of the eye. The cornea
begins the process of refraction by bending the light
rays that pass through it. The light rays pass through
the pupil of the eye, which is a hole that expands or
narrows depending on the amount of light entering it.
Light then travels through the crystalline lens of the
eye that narrows and refines the rays even more so,
producing clearer images and focusing of images close
up. This process is called accommodation and decreases
with age. The result of the eye's decrease in accommodation
is a condition called presbyopia and results in the
need for reading glasses to see things close up. Light
then passes through the transparent fluid in the inner
eye, which also helps to focus the light rays even further.
The light finally reaches the retina where it is sent
to the brain.
Nearsightedness (Myopia)
If light rays fall in front of the retina because the
curvature of the cornea is too steep, vision will be
blurry when objects are seen at a distance. This condition
is called nearsightedness (myopia) and is remedied by
the use of eyeglasses or contact lenses that assist
the cornea in bending light rays so that they fall correctly
on the retina.
Farsightedness (Hyperopia)
If light rays fall behind the retina because the curvature
of the cornea is too flat, vision will be blurry when
objects are seen at a distance and close up. This condition
is called farsightedness (hyperopia) and is remedied
by the use of eyeglasses or contact lenses that assist
the cornea in bending light rays so that they fall correctly
on the retina.
Astigmatism
The shape of the eye is usually spherical much like
that of a basketball. If the eye is shaped more like
a football then the images do not fall correctly on
the retina and appear distorted. This condition is called
astigmatism and may be modified or corrected with the
use of contact lenses or eyeglasses. Although it may
sound disconcerting, astigmatism is quite common and
usually is accompanied by some amount of hyperopia,
or myopia.
We use a computerized device called an autorefractor
to obtain a close estimate of a patient's refractive
error. This device utilizes a sensitive scanning laser
to read the focusing capacity of the eye. The information
is printed out and the technician and doctor will use
it in their further evaluation of the patient.
All three refractive errors, myopia, hyperopia and
astigmatism are correctable with refractive surgery.
Please refer to the
LASIK section of our website for additional information.
