How Does Vision Work?
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How do your eyes and brain work together to turn light into images? You'll learn about the working components of the eye and take a look at two theories of color perception.
We humans rely on our senses of sight more than a lot of other animals. A cat can easily find its way in the dark thanks to its sensitive whiskers, but if you try closing your eyes and trying to find your way from your desk to the office kitchen, you'll probably hurt yourself (or others). The smell of microwavable entrees doesn't guide you with precision, nor can you pinpoint the sounds of conversation accurately enough to keep you from running into the wall outside. We perceive the world largely through our eyes.
Still, our eyes are still limited in the grand scheme of things; they can perceive only a tiny fraction of the light that makes up the full electromagnetic spectrum. Light can have many different wavelengths and different properties at these different wavelengths. X-rays have really short wavelengths and can be used in medicine to 'see through' the body. Microwaves have much longer wavelengths and heat up your food when you're feeling lazy. We can't see x-rays or microwaves; we can only see light that has wavelengths within what's known as the visible spectrum. The longer wavelengths of the visible spectrum produce red light, and the shorter wavelengths produce purple light. If you've ever seen a rainbow, or light that's been passed through a prism, you've seen the full range of the visible spectrum in order from longest to shortest wavelength.
The light in the visible spectrum really isn't any different than the other kinds of light. We only call it the visible spectrum because it's the range that human eyes have evolved to be able to perceive. If birds had defined a visible spectrum, it would include ultraviolet light, or wavelengths that are just a little shorter than what humans can see. Birds see ultraviolet light just like any other color, because their eyes have evolved a little differently than ours.
So we've talked a lot about what our eyes perceive; now let's take a closer look at how our eyes take in light and give us the world of images and color we're used to seeing.
Light first passes through the cornea, the outermost part of our eyeball, where it begins to be focused. Then it enters the pupil, a small opening that leads to the lens. Take a look in the mirror; the pupil is the black part at the center of your eye. The colored part around it is called the iris, and it grows and shrinks to protect the pupil and make sure the right amount of light gets in. On a really bright day, your pupils appear to shrink as your irises block more of them; in a dark room, your pupils appear to grow as your irises retract to allow in as much light as possible. You've probably noticed that when you turn out your light to go to bed, your room seems really dark; eventually, your eyes adjust and you can see enough to stumble to the bathroom without running into your desk. Your irises retracting to allow more light into your pupil allows you to do this.
Once the light makes it past the pupil, it hits the lens, a surface where it is further focused through a process called accommodation. If you need glasses, it's probably due to problems with accommodation; if your lenses won't do it naturally, light needs to first pass through artificial lenses--either glasses or contacts--in order to be properly focused.
Finally, the light hits the retina, a layer of tissue that lines the inner part of the eye. The retina begins the process of turning the light into an image. The retina contains two kinds of photoreceptor neurons, known as rods and cones. Rods are located more at the edge of the retina and process black and white; cones are in the interior and process color and image details. Exactly how color is processed has gone through some debate.
19th century scientists Thomas Young and Hermann von Helmholtz developed the Young-Helmholtz theory of color perception, which proposed that there are three different kinds of cones that process different colors of light--one for blue, one for green and one for red. Other colors are made up of mixtures of these. While this theory elegantly explains color-blindness (people who are red-green colorblind, for example, would be missing those kinds of cones), it does not quite explain why these people are able to see yellow, for example, which would need to be processed by both these kinds of cones.
Another more recent theory is the opponent-process theory, which holds that we don't process colors as a mix of three primary colors but instead as three sets of opponent colors: red-green, yellow-blue and white-black. Evidence for this model can be found in the phenomenon of after-image. If you stare for a long time at something red then look at a white surface, you'll see a green after-image.
Signals from the rods and cones pass through a bundle of neurons called the optic nerve on their way to the brain. The retina's process of converting electromagnetic light waves to electronic stimuli is called phototransduction. The brain processes and merges the signals from both eyes to produce a properly oriented, three-dimensional image. Certain feature detector neurons in the visual cortex can isolate things like lines and shapes in order to help us quickly interpret what we're seeing.
Vision, humans' most important sense, involves a complicated process of converting light signals into images in the brain. Light passes through the lens, where it is focused, to the retina where photoreceptors called rods and cones convert the information to electrical impulses that can be interpreted by the brain.