I saw a double rainbow as I was driving in Wisconsin a few weeks ago. How is this phenomenon formed?
A rainbow forms when sunlight is refracted twice, first when entering a raindrop and then when exiting it, and reflected once within the drop. Sunlight enters a raindrop on the side of the drop facing an observer and is separated into its component colors (the first refraction), then reflects off the back side of the raindrop and refracts a second time (again on the side of the drop facing the observer) when leaving the drop. Colors range from red on the top of the rainbow through orange, yellow, green and violet at the bottom.
A secondary (or double) rainbow forms when sunlight is reflected twice inside a raindrop and the sequence of colors is reversed. The secondary appears above the primary and is much fainter because it is spread over a greater portion of the sky and it contains less light from the second reflection.
Rainbows also result from the mist thrown into the air at the base of waterfalls and even from backyard water sprinklers.