Re: What are the colors of the rainbow? Why can't you see some if them?

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.