MadSci Network: Astronomy

Re: I have a theory, just wanted a scientist's opinion.

Date: Sat May 17 17:51:22 2003
Posted By: Benjamin Monreal, Grad student, Physics, MIT
Area of science: Astronomy
ID: 1047945558.As

Hello Johnathan,
That's an interesting set of thoughts. I think we can clarify things a bit by talking, not about color, but light.

Color is a very complicated phenomenon, but it amounts to how our brain perceives different wavelengths of light. Red light has a long wavelength, blue light has a shorter one, yellow is in between. Most color consist of mixtures of many wavelengths; broadly speaking, light with a single wavelength will have a very "saturated" color, and light with a mixture of many wavelengths will be "unsaturated". The total amount of light, added up over all wavelengths, is seen as "brightness". Some examples: a red laser pointer emits a single pure red wavelength; so its color is red, its saturation is high, and its brightness can be high or low, depending on how nearby you are. The light waves coming from a piece of red paper are mostly long-wavelength, so its color is red; there is a mixture of other wavelengths too, though, so it is less saturated than the laser pointer; a brown or pink would be even less saturated. The brightness of the paper depends on how brightly lit it is.

Anyway, so let's talk about black objects, white objects, and mirrors. Something we see as "black" is something with a very low brightness---there is not much light at all reaching our eyes from this object. (It's hard to say, then, that black is a "color" - if there is no light at long, short, or medium wavelengths, there's no point in defining color!) And of course brightness is a matter of perception. Go outside on a moonlit night, and you might describe everything as being "black". But if you set up a camera on a tripod, and take a very long exposure photograph, the camera will very easily see the green of grass, brown of wood, blue of lilacs. So, black is the absence (or scarcity) of light and unclear to define in terms of color.

White can be defined in terms of color, since there are light rays present and we can measure their wavelength. White basically means that there are equal amounts of light at all wavelengths. Usually we like to also say white has a high "brightness", to distinguish it from gray. But the defining characteristic of "white" is the even mixture of long, medium, and short wavelengths. In this sense, you could again say that white is not a color, but rather the maximum-unsaturated version of any color.

OK, I've just described white light, let's talk about white objects. The color of an object is just the color of the light rays coming from that object and hitting our eyes. These light rays depend on two things: what wavelengths of light are hitting the object to start with? And, of these wavelengths, which ones are getting reflected into our eyes? For a "black" object, no matter what wavelengths are hitting it, they will be absorbed---thus no wavelengths reach our eyes and we call it black. For a "saturated, red" object, it will reflect red wavelengths and absorb all others. Assuming that some red wavelengths are available in the ambient light, the object will reflect red, and we see it as red. And so on. Note that a "red" object will appear black if illuminated in purely blue light, for example. The only thing that matters is what wavelengths hit your eyes. A white object will reflect any color that hits it! We're used to having generally-white light bulbs and suns, so most white objects indeed reflect this common white light!

OK, finally we get to mirrors. Mirrors (well, most household mirrors) reflect all wavelengths - just like a white object. The difference is that mirrors preserve the original angle of the light, whereas most objects will scatter it in all directions. If you illuminate a mirror with white light, then indeed white light will bounce off of the mirror---the reason the mirror looks so strange is that the light only bounces in a particular direction.

So, if I had to assign a "color" to a mirror, I'd say it is not black, but white! In practice, the best way to describe what is going on is this: noting that a "white" object will reflect whatever color is hitting it. When looking at a mirror, it indeed reflects whatever color is hitting it---but because of the directionality, a different color (whatever colors are in the reflected scene) are hitting it at every point!

Anyway. As you can see "color" is a complicated concept, first explored perhaps by Goethe in his "Theory of Colors", later by Newton in his "Optics." Nowadays we understand light waves, emission, and absorption very well---more complicated, though, is understanding color perception in the human eyes and brain. This is the science (and art) of color perception, which you can read about on the web.

Hope this helps,
-Ben Monreal

Current Queue | Current Queue for Astronomy | Astronomy archives

Try the links in the MadSci Library for more information on Astronomy.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2003. All rights reserved.