How does your eye see things?
There are two parts to seeing: first, an image of the world is formed on
a sensitive surface, and second, the image on the sensitive surface is
interpreted by your nervous system. The first job is done by your eye, a
little hollow ball with a sensitive surface (the retina) at the back and a
hole (the pupil) at the front. Now light rays from lamps or the sun bounce
off all objects around you, and these light rays come flying off these
objects in all directions. If you make a tiny hole in a box, this
"pinhole" throws away all the light rays that hit the box *except* those
that draw a perfect little image of the world on the back inside surface.
You can prove this: draw a tree and a dog below it, and a box facing them.
Draw straight lines coming out from the tree and dog in all directions and
let them pass through the front of the box. You'll see that those that hit
the back of the box from the tree and the dog are all mixed up together.
Now if you only let through the front of the box those rays that pass
through a tiny hole, you'll see that the rays are sorted out, so that those
from the dog hit a different place on the back than those from the tree.
You've got an image! Notice the image is upside down.
You can make a camera or an eye with just a box with a pinhole and a
sensitive surface (a retina or film), but a *lens* is better than a
pinhole. A lens bends the light from a whole patch of rays hitting the
barrier so that they all go through the pinhole. You end up throwing away
fewer rays and you get a brighter image. Now the rays hit the box at
different angles, depending on how far away the objects are that they came
from, as you can see from your drawing if you draw a tree farther away.
And a lens bends rays a fixed amount, so the lens can only "focus" (pass
through the pinhole) rays from objects at one distance. You can change the
focus, however, by either moving the lens toward or away from the pinhole
(how a camera does it) or by changing the shape of the lens (how your eye
does it). You have two lenses in your eye: the first is the outer surface,
called the cornea, and the second is the lens proper. The cornea is fixed
in shape, but the lens is flexible, and anchored to a circular ring of
muscle. When this muscle contracts, it flattens the lens, and the eye
focuses on objects farther away (the bending power of the lens is reduced
when it's flatter and rays coming from farther away are bent properly).
When the iris relaxes, the lens springs back into a rounder shape, and you
focus close up. As one gets older, the lens gets less springy, your
ability to focus close gets worse, and you need reading glasses.
Now what does the retina do with the image? The first layers of cells in
the retina is composed of four types of light sensitive cells: "Rod" cells
respond to total light levels while "cone" cells come in three varieties
that each respond to the levels of red, blue or green light. The rod cells
are more sensitive, but they don't respond differently to different colored
light, so that's why in the dark, when only rod cells are working, you
can't see colors. The cone cells work together to let you see colors other
than red, blue and green. You see yellow light as yellow because it
stimulates red and green cells equally and your brain interprets that as
orange. But you can fool the brain, because you can shine equal amounts
red and green light on your eye, and you'd *still* see it as yellow, even
though it isn't. That's why color TV is possible. The station only needs
to send to you a red, green and blue image, and by mixing up the strengths
of each color in the right way they can fool your brain into seeing all
kinds of colors that aren't really there, like purple and puce and pink.
Now each cell contains a "visual pigment," a chemical that changes its
shape when light hits it. The changed shape of the chemical triggers an
electrical impulse that the light-sensitive cell sends to the next two
layers of cells in the retina. These cells are hooked up to patches of the
light-sensitive cells, and they do several things. The first thing they do
is provide a sensitivity control on the light-sensitive cells, so you can
see in very dim light (candlelight) through very bright light (daylight on
snow, which is ten million times brighter). They also analyze the image.
Certain groups of these cells respond to shadow edges in an image, to rapid
motion, for fine color detail, and so on. All of this information is
gathered by the optic nerve and transmitted to the "visual cortex", the
back surface of your brain, and there in a way we still don't understand
your mind responds to all this information with a sensation of an image.
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Update: June 2012