h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

... under a rainbow lighter than above it? I am not sure I notice this in realtime, but on pictures taken of rainbows, this effect is always there. So why is it?

Here's everything you need to know about rainbows.

http://eo.ucar.edu/rainbows/

"Why is the sky brighter inside a rainbow?

Notice the contrast between the sky inside the arc and outside it. When one studies the refraction of sunlight on a raindrop one finds that there are many rays emerging at angles smaller than the rainbow ray, but essentially no light from single internal reflections at angles greater than this ray. Thus there is a lot of light within the bow, and very little beyond it. Because this light is a mix of all the rainbow colors, it is white. In the case of the secondary rainbow, the rainbow ray is the smallest angle and there are many rays emerging at angles greater than this one. Therefore the two bows combine to define a dark region between them - called Alexander's Dark Band, in honor of Alexander of Aphrodisias who discussed it some 1800 years ago!"

It's all done with mirrors.

PP.

If you applaud, the rain will take a bow.

Now I can't get "Alexander's Ragtime Band" out of my head!