This is the rainbow we are all most familiar with. The primary rainbow is the single multi-colored arc that usually appears after a rainstorm. Primary rainbows are formed when refracted light is reflected through a water droplet. The intensity of the rainbow’s colors depends on how large the water droplets are.
If you have seen a primary rainbow, then chances are you have also seen a secondary rainbow. They are also known as double rainbows. A secondary rainbow forms behind the primary rainbow when the light in the water droplet is reflected twice instead of once. The secondary is about twice as wide as the primary rainbow, but is only one-tenth as intense. Its colors are also reversed.
Alexander’s Dark Band
Alexander’s Band is technically not a rainbow, but it is associated with the primary and secondary rainbows. An Alexander’s band is the area of sky between the primary and secondary rainbow and it is noticeably darker than the rest of the sky. The single reflected light of the primary brightens the sky inside and the double reflected light of the secondary brightens the sky outside of it. To our eyes, it appears that the sky is darker between the primary and secondary rainbows.