Primary and Secondary Rainbows
Sometimes we see two rainbows at once. What causes that? Not all of the energy of the ray escapes the raindrop after it is reflected once. A part of the ray is reflected again and travels along inside the drop to emerge from the drop. The rainbow we normally see is called the primary rainbow, and its produced by one internal reflection; the secondary rainbow arises from two internal reflections and the rays exit the drop at an angle of 50 degrees rather than the 42 degrees for the red primary bow. Blue light emerges at an even larger angle of 53 degrees. Its effect producesa secondary rainbow that has its colors reversed compared to the primary.
Figure
4. Source:
http://www.home.earthlink.net/~nwwallace/rainbows.htm
Red light from the high droplets in the group will reach the observer, but the green and blue light will be directed above the observer's eye. Green light from the middle of the group will reach the observer, as will blue light from the lowest droplets in the group. The observer will then see a rainbow, red on top, green in the middle, blue or violet on the bottom.
It is possible for the light to be reflected more than twice within a raindrop, and one can calculate where the higher order rainbows might be seen; but these are never seen in normal circumstances.
