Why do we see rainbows, how do they work?

-We see rainbows when the sun is behind us and falling rain is in front of us.

-When sunlight strikes a falling drop of water it is refracted, changed indirection, by the surface of the water.
The light continues into the drop and is reflected from the back of the drop to the front.  When the beam hits the front it is refracted again and emerges from the drop as the color spectrum that we see in a rainbow.

-The water drop acts like a prism to seperate the light into its different wave lengths.

DOUBLE RAINBOW-If the beam is reflected twice inside of the water drop them it will cause a secondary rainbow to appear when the light leaves the water drop.  The colors of the secondary 
rainbow are reversed in order with violet
on the top and red at the bottom.



As an example of a double rainbow.
The faded rainbow on the right is the secondary rainbow.
You can see the reverse in colors on the secondary rainbow compared to the primary rainbow.