What causes Rainbows?
The cause of this optical phenomena is light interacting with water droplets in a region where rain is falling. The person seeing the rainbow must be standing in front of the sun and behind the rain drops. The sun passes over the person and strikes the rain droplets and then comes back to the person.
When the sunbeam enters a raindrop, it refracts at the front surface of the drop into a spectrum of colors. The various colors have different rays which all head in slightly different directions. The colored rays continue into the drop, heading toward the front. When the spreading beam hits the front surface for the second time, it refracts out of the drop, spreading the beam into a wider spectrum that is headed toward the person observing the rainbow. A rainbow occurs when a large group of rain droplets are refracting light in this way.
http://www.cs.princeton.edu/~rywang/berkeley/magic_small/rainbows.html
Red light from the high droplets in the group will reach the observers, the green and blue light will be directed above the observer's eye. Green light from the middle group of rain drops will reach the observer, as will blue light from the lowest drops in the group refracting light. So in the end, the person will see red light on top, green in the middle, and blue or violet on the bottom.