HOW
DO WE SEE?
 The
eye functions like a camera or more like a video camera. Light rays
coming from the objects arrive on the cornea first. The cornea provides
70% of the refractive power of the eye. After refracting the light rays,
it directs them to the lens through an aperture called the pupil. The
lens bends, in turn, refracts the rays again. It makes a fine adjustment
to focus the light rays on the retina. This adjustment is similar to
the auto-focus function of cameras. This superb, silent and continuous
auto-focus function is due to the lens' ability to change its refractive
power by altering its form and thickness. This function is called accommodation.
In a non-accommodating young eye the refractive power of the lens is
below 20 diopters but with maximum accommodation it increases to more
than 30 diopters, which is nearly equal to an 8.5 diopters increase
in the eye's total refractive power. Because of this ability the young
eye can see both near and far objects well.
After the light rays are bent at the lens, they pass through the vitreous,
the jelly-like substance filling the back part of the eyeball, and reach
the retina. The retina, which makes up the inner lining of the eyeball
functions like a camera film, or a CCD of a digital video camera. Then
light rays are converted into electrical impulses by the retina and
carried to the brain by the optic nerve. These impulses are processed
and perceived as images by the brain.
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