The eye works like a
camera. The light rays of the
image enter the eye through the
cornea, a clear window
similar to a lens filter, which provides
most of the focusing power of the eye. The
cornea is composed of several layers of
tissue. The outer layer or epithelium is
the eye's protective layer. This layer is
made up of cells that have the ability to
grow back within five to seven days, and
therefore, allow for fast healing of
superficial injuries. Most of the inner
layers provide strength to the eye. The
middle inner layer, the stroma, is the
largest layer and the part of the cornea
that is typically modified in refractive
surgery to change the focus. The last
layer is the endothelium, a very important
layer that is largely responsible for
keeping the cornea clear.
After
the cornea, the partially focused image
then travels through the
pupil. the pupil is
the "black circle" that you see in
people's eyes. The iris, the colored
part of the eye (ie. blue, green,
brown, or hazel) determines the size of
the pupil. The primary function of the
iris is to control the size of the
pupil and therefore the amount of light
entering the eye. This is achieved
through contraction or expansion of the
muscles of the iris. When you are in a
bright environment, the iris contracts
to allow less light through. when it is
dark, the pupil expands to allow more
light to reach the back of the
eye.
The
lens, the next element
in this optical system, is a clear
structure located just behind the
pupil. Its primary function is to
provide fine-tuning for focusing and
reading. the lens performs this
function by altering its shape. At
about the age of 40-50, the lens
becomes less flexible as
presbyopia sets in.
Presbyopia, or loss of near vision is
why many of us who never had to wear
glasses before, need them to read with
after forty years of age. Finally,
sometime around age 60 to 70, the lens
becomes cloudy and hard (cataract
formation), preventing light from
entering the eye. These cataracts may
then be removed with advanced
techniques.
The lens
fine-tunes the image to focus it
properly on the
retina. The retina is
a thin layer of nerve tissue that lines
the inside of the eye and functions
like the film in a camera. The retina
transforms the image into electric
impulses that are then carried by the
optic nerve to the brain transforms the
light. For you to see clearly, light
must be focused precisely on the
retina. Glasses or contact lenses are
required when your eye cannot focus
light properly.
