Skim the first paragraphs of the Wiki articles, below. What we can see is just one part of electromagnetic waves of all sizes.
The part we can see is … visible. Dah-dah!
But it gets plenty interesting. Not everyone sees the same thing! And some animals can see radiation that we cannot. Pictures of flowers the way “bees see them” show wild and unexpected patterns. http://www.google.com/search?sugexp=chrome,mod=14&sourceid=chrome&ie=UTF-8&q=how+bees+see+flowers
Our ability to see comes from four receptors in our eyes. One of them only sees in black-and-white, and that’s especially good for night vision. (Notice how all the colors disappear at night?) Those black-and-white loving “rods” are the reason that black-and-white photos, and black-and-white video doesn’t look completely wack — because part of our brain understands those monochrome images very well indeed.
It’s the cones that see color, and there are three kinds of them. One that likes blue, one that likes greens, and one that likes reds. That’s the reason blue, green and red are called “primary colors” … they aren’t really “primary” to anything, except the way our eyes see the radiation.
People who are color blind tend not to be able to distinguish, for example, between “primary” colors such as green and red. To a color blind person, they look much the same.
So finally you’re probably wondering how cool it would be to have extended vision such as bees. Or to even be able to see radiation like Geordi on Star Trek. There are three factors that would tend to limit how useful this is. Some of the other radiation is really fuzzy — our evolution picked radiation that produces sharp images. Also, some of that radiation is dangerous — an animal that was regularly exposed to it would die. Also, some of that radiation has such a long wavelength, even with much bigger eyes and rods and cones we wouldn’t be able to see it.
Finally, human beings can already distinguish something like 10,000,000 colors. That’s far more than enough for most circumstances, and in practice most people cannot easily distinguish between ones that are very close.
But this whole subject does get one thinking, doesn’t it? For example, consider what colors a digital camera “sees”.
The visible spectrum is just one segment of the entire range of electromagnetic wavelengths. The shortest wavelengths in the electromagnetic spectrum are gamma rays–invisible to us, but extremely dangerous when we are exposed to them. Radiowaves are the longest wavelengths. Somewhere in the middle is the visible range of light: violet, blue, green, yellow, orange, and red. These wavelengths are still pretty small, from 400 to 700 nanometers in length, but as freedsmooth mentions, our eyes can still pick them out.
Both the visible and invisible ranges of the electromagnetic spectrum are used in remote sensing technologies–the satellites and other forms of aerial photography that capture images of our earth from afar.
The visible spectrum is a very small portion of electromagnetic spectrum that is visible to the human eye.
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