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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.

Christopher Grayce

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