Why is the sky always lighter under a rainbow?

Double-alaskan-rainbow

The sky inside of a primary rainbow is brighter than the sky outside of the bow. This is because each raindrop is a sphere and it scatters light in a many-layered stack of coloured discs over an entire circular disc in the sky, but only the edge of the disc, which is coloured, is what is called a rainbow. Alistair Fraser, coauthor of ‘The Rainbow Bridge: Rainbows in Art, Myth, and Science’, explains: “Each color has a slightly different sized disc and since they overlap except for the edge, the overlapping colors give white, which brightens the sky on the inside of the circle. On the edge, however, the different-sized colored discs don’t overlap and display their respective colors — a rainbow arc.” Via Rainbow

rainbow Rainbow picture by Alan Carr

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