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How the Eye Works 
To appreciate how
the eye works, imagine yourself as a beam of light reflected from a
100-foot tree into the eye of a living person. You are the image of
the tree, traveling at the speed of light and about to enter an
obstacle course on your way to the brain of the observer
Your
first encounter is your passage through the clear convex cornea
which bends (refracts) you and slows you down. It also shrinks you
to a manageable size (a little larger than a nickel). Next you
squeeze through a round, adjustable opening, the pupil, formed by a
colorful membrane, the iris, which, if you are too bright, will
reduce your intensity.
You now encounter a rather dense but
transparent medium, the lens, which not only bends you even more,
but unceremoniously turns you upside down. It then aims (focuses)
you at the back of the eye, the retina, which you strike after
passing through a clear, sticky, gel-like substance, the vitreous
humor.
You are now the inverted image of a 100-foot tree
shrunk to the size of a postage stamp and flattened against the
retina. But not for long! Instantly you are transformed from a beam
of light into an electrical impulse, and flashed along the optic
nerve from the retina to the brain. You are now perceived as a
100-foot, three-dimensional, right-side-up tree. And all of this in
a tiny fraction of a second.
To summarize how the eye works:
the cornea is the clear, transparent front covering which admits
light and begins the refractive process It also keeps foreign
particles from entering the eye.
The pupil is an adjustable
opening that controls the intensity of light permitted to strike the
lens. The lens focuses light through the vitreous humor, a clear
gel-like substance that fills the back of the eye and supports the
retina.
The retina receives the focused image from the lens,
and transforms this image into electrical impulses that are carried
by the optic nerve to the brain.
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