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HOW AN EYE 
WORKS:
The eyes do not actually see objects -- what they see is
the light that objects reflect. When these light rays
enter the eye, they are absorbed and converted into
electrical signals by retinal nerves. These signals are
subsequently sent to the brain where they are interpreted
as visual images. In a normal eye, light rays enter the
eye through the "cornea" (window of the eye)
and are focused with the help of a "crystalline
lens" behind the pupil at a point directly on the
retina (the light sensitive nervous tissue at the back of
the eyeball). However, only about four in ten people have
normal visual acuity. For the rest, clear vision may be
achieved by refocusing light rays with the use of
corrective lenses.
HOW A
CONTACT LENS WORKS:
Contact lenses are delicately-crafted, very thin, small,
optical discs , worn directly on the eye. They are
comfortably held in place by a natural layer of tears
present between the contact lens and the cornea. Contacts
eliminate the eyeglass barriers that interfere with the
line of sight above, below and to the sides of the eye,
offering outstanding peripheral vision. In addition,
contacts can reduce or eliminate the image distortion
sometimes caused by eyeglasses.
There are
several types of contact lenses designed to fit the most
common vision conditions. Following are brief
descriptions of these conditions:
NEARSIGHTEDNESS
(MYOPIA):
Myopia is a condition which occurs when the eyeball is
too long, or the eye's focusing mechanism is too powerful
(cornea and
 lens), and light rays
are focused in front of the retina. People with this
condition can see clearly up close but not at a distance.
Lenses to correct this common condition are thinner in
the center than on the edges to help redirect light rays
to the retina, and are called "minus" or
concave lenses.
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FARSIGHTEDNESS
(HYPEROPIA):
Hyperopia occurs when the eyeball is too short from front
to back, or the eye's focusing mechanism is too weak,
causing light rays to be focused behind, rather than on
the retina. People with hyperopia have difficulty seeing
objects close up.
 In
order to correct this vision problem, a convex, or
"plus" lens is prescribed. This lens is thicker
in the center and thinner on the edges.
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ASTIGMATISM:
Astigmatism is characterized by an irregularly-shaped
cornea which causes light images to focus on two separate
points in the eye, creating a distorted image. Symptoms
range from visual discomfort in mild cases, to severe
blurring and distortion similar to a reflection in a
fun-house mirror. Contact lenses designed to provide
astigmatic correction are fitted for each individual. The
misshapen cornea is precisely measured and special toric
lenses are used to direct light rays to one spot on the
retina. There are more than 60 thousand different toric
prescriptions available, offering the many people
affected by astigmatism an alternative to eyeglasses.
Rigid Gas Permeable (RGP) lenses, due to their firm
design, offer a high degree of corrective ability for
this condition, and in many cases, do not require a
complicated toric design due to their ability to
compensate for the irregularly shaped cornea.
| An
astigmatic eye causes the light to focus at
different points resulting in blurred or
distorted vision.
ASTIGMATISM'S
TEST . Close one eye
and then the other one , if you do not see all
the lined squares, in the same black color, or if
you see one or more squares greyer than the
others, you than have an astigmatism. Consult
your optometrist to examine your eyes to
determine the amount of astigmatism that you
have, and if it is affecting your lifestyle.
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PRESBYOPIA:
This is a condition that occurs as the eye's lens grows
older and thicker, and the eye muscles cannot act so
effectively in altering its shape. It then becomes harder
to switch focus between viewing near and far objects, and
reaches a point where reading anything up close becomes
very difficult. Also known as "ageing
eye," presbyopia actually starts at about age 10,
most people do not begin to experience the effects of
presbyopia until their forties. Half-glasses or
bifocals used to be the only answer for people with
presbyopia. Today there are a number of contact lenses
that can correct this condition, including multifocal
contacts and specialised fitting techniques such as monovision. In
addition a new surgical technique called the Surgical Reversal of
Presbyopia (SRP) is under clinical
trials in the US.
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