Rainbows are really weird, when you stop and think about it. To a certain extent they don’t even exist – any given rainbow is a mass optical illusion rather than an actual thing.
But they are fascinating. Even the sky around them is fascinating: Lighter on the inside, darker on the outside, darker still between double rainbows.
Why? Optics. Meaning: It’s complicated. Here’s one discussion about it.
I mention this because Geoff Forester, The Concord Monitor’s long-time photo chief, took the above astonishing photo, which shows the lighter-below phenomena to the point that I thought it was filtered. He swears it wasn’t.