What causes double rainbows?

We have discussed how the refraction of light entering a raindrop creates the different colors of a rainbow. However, the inside of the drop is more complicated than you may think.

The light can also reflect off the back of the raindrop and then exit the raindrop in a different location; so not all the light exits at that point, but some of this visible light is reflected a second time. After the second reflection, the light will leave the drop at a different angle. This causes the creation of a second rainbow. The first one (on the inside) is called the primary rainbow and the outside one is called the secondary rainbow.

It is even possible for the light to be reflected many more times within a raindrop, but one rarely ever sees more than two rainbows.