General Knowledge
What causes the fainter second bow in a double rainbow?
- Light reflecting twice inside a drop
- Two layers of cloud
- Two light sources
The answer is Light reflecting twice inside a drop. A second internal reflection - and its colours run in reverse.
One extra bounce. In the primary bow, light enters a raindrop, reflects once off the back, and comes out. In the secondary, it reflects twice before leaving. Each reflection loses light, which is why the second bow is always fainter, and the extra bounce turns the sequence over - so the primary runs red on the outside and the secondary runs red on the inside.
The geometry also fixes their positions exactly. The primary sits at about 42 degrees from the point directly opposite the sun, the secondary at about 51. Between them is a band that is noticeably darker than the sky on either side, because no light at all is returned at those angles. It has a name - Alexander's band, after Alexander of Aphrodisias, who described it around AD 200.
Both bows are circles, and you only see the top of them because the ground is in the way. From an aircraft, with the sun high behind you, a rainbow can appear as a complete ring around the shadow of the plane.