The USGS Water Science School
Have you ever seen six rainbows at once? They are not only rare to see—they are a puzzle to understand. The common rainbow is caused by sunlight being reflected by the backs of falling raindrops, while also being refracted at the air/water boundary. The sunlight in this picture is coming from behind the observer, and the rainbows are in the rainstorm.
The brightest rainbow is the primary rainbow. Above and to the left of the main rainbow is a secondary rainbow, caused by multiple internal reflections inside water droplets, with colors reversed. Harder to explain is the intermediate rainbow, between the two. This rainbow is likely caused by sunlight that has first reflected off the lake before striking the distant raindrops, which then reflect sunlight back toward the observer. That accounts for the three rainbows in the sky, and the other three are reflections of the rainbows in the sky on the lake's surface. Kind of cheating to say there are six rainbows, but why argue with such a spectacular show? (NASA).
You're probably wondering the same thing that I'm wondering—why are the colors of the secondary rainbow reversed? And why is there a secondary rainbow at all? After researching this I still can't understand it, so below the picture is an explanation from the National Center for Atmospheric Research and the UCAR Office of Programs:
Photo/copyright: Terje O. Nordvik
Explanation from the National Center for Atmospheric Research and the UCAR Office of Programs:
Sometimes we see two rainbows at once, what causes this? In a rainbow, a ray of sunlight enters and is reflected inside the raindrop. But 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 is 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, this effect produces a secondary rainbow that has its colors reversed compared to the primary.