|MadSci Network: Physics|
A rainbow is formed when visible light, or "white light" gets broken into its different wavelengths by a spectrum (or, in the case of a rainbow in the cloud, by raindrops, which act the same way). Usually we see all the wavelengths of light at one time, and they all blur together into white. A prism focuses the light, the same way a projector's lens does, and then we can see the different colors. They are, starting from long wavelengths and going to shorter ones: RED ORANGE YELLOW GREEN BLUE INDIGO VIOLET. These are the colors we can see. The "colors" we can't see really are not colors -- after all, we can't see them! On the left of red (at very long wavelengths) we have *infrared*; to the right of violet (at very short wavelengths) we have *ultraviolet*. We can't see them, but we can see the effect they have on things: infrared warms things, while ultraviolet makes certain things glow. That, in essence, is how we know they exist, even if we can't see them. Now, as for why we can't see them...well, that's something nobody really knows. There are some creatures who can see them, though. Bees, for example, can see colors in the ultraviolet spectrum.
Try the links in the MadSci Library for more information on Physics.