Why do rainbows happen?
When the sun is behind you and rain is in front of you, each raindrop bends sunlight, bounces it off its back and bends it again on the way out. Different colors bend by different amounts, so the light fans out into a band of colors about 42 degrees around the point directly opposite the sun.
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Every drop is a tiny prism
Light slows down and bends when it enters water. Blue and violet bend a little more than red, so white sunlight separates into colors. Inside the drop the light reflects off the back and exits toward you, at about 42 degrees for red and about 40 for violet, which is why red is on the outside of the arc.
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Why it's an arc, and why you can't reach it
You see color from every drop that sits at that angle from the point opposite the sun, which traces out a circle around your shadow's head. The ground cuts off the bottom, so you usually see an arc. Everyone sees their own rainbow from a different set of drops, and it moves as you move, so you can never reach the end.
Double rainbows and best times
A fainter second rainbow forms outside the first when light reflects twice inside the drops; its colors are reversed. Rainbows are most common in the morning and late afternoon, because the sun has to be lower than about 42 degrees for the arc to appear above the horizon. Showers moving away while the sun breaks out behind them are the classic setup.
Questions people ask
- Why is the sky brighter inside a rainbow?
- Drops inside the arc also send light toward you, but with the colors mixed together, so the sky inside looks brighter.
- Can you see a rainbow at noon?
- Usually not in summer, when the sun is too high. In winter, or from a plane or a mountain, you sometimes can.