Double Rainbow at Dusk
Unusual double rainbow on CA Route 1. Note: the colors in the double are lighter and reversed. We get rainbows when sunlight shines directly into a rain drop, then bounces off the back of the raindrop, and is reflected to your eyes. The water acts like a prism and spreads the light into the full spectrum of colors. When a secondary bow is formed, the light has to bounce twice off the back of the rain drop before it is reflected to your eyes. That means more light is muted by the rain drop (the cause of it being less bright) and like a mirror the colors are flipped backward to the primary rainbow since the light bounces off one more surface of the raindrop. So every time the light bounces off the back of the drop, the reflection, much like a mirror, is flipped and reversed. This is same thing as when you look in a mirror everything looks backwards.
From On The Road Again