How are double rainbows formed?
A rainbow forms when sunlight is refracted twice (first upon entering a
raindrop and then when exiting it) and reflected once within the
raindrop. 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. The 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 raindrops 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
Rainbows also result from the mist thrown into the air at the base of
waterfalls and even, occasionally, from water sprinklers on lawns.