When direct sunlight strikes falling rain, a rainbow is seen at a point directly opposite the Sun. A double rainbow occurs when some of the light entering the raindrop is refracted into its component colors, reflected off the back interior wall of the drop, and refracted again as it exits the drop. Click on image for full size (29 Kb) Image Courtesy of Carlye Calvin/University Corporation for Atmospheric Research

You can only see a rainbow if the sun is behind you and the rain in front. You can even make your own rainbow with a garden hose or water sprinkler on a sunny day.

A double rainbow occurs when some of the light entering the raindrop is refracted into its component colors, reflected off the back interior wall of the drop, and refracted again as it exits the drop. In the second rainbow, the colors are reversed where blue is on the outside and red is on the inside. The dark area in between the two rainbows is called Alexander's band.