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Q How do the sun and the rain make rainbows?

— Stella Balsley

A Steve Ackerman, professor of atmospheric and oceanic sciences at the University of Wisconsin-Madison:

Rainbows required a number of scientists to actually explain how they form, including Isaac Newton.

Light from the sun is made of multiple colors, not only the colors we can see, but also the colors that we can’t see. Newton demonstrated this by passing a ray of light through a crystal. As the light passes through the crystal, the light bends, and how much it bends allows the colors to separate.

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The other thing we need to know is that calm water can act like a mirror. Now let’s put all this together and talk about how the rainbow forms.

What happens is a ray of light hits an individual raindrop. As it goes in, it bends. That bending is a function of the color.

If it goes in at the right angle, it hits the back of the drop, and the back of the drop acts like a mirror and reflects that ray of light back toward the front. If it leaves the drop into the air, it bends again. So those colors get separated. And eventually it will reach our eye.

But only one color will reach our eye from every single drop. So the rainbow effect when you see the entire bow or even part of the bow — that’s not coming from one drop. That’s coming from millions of drops. Every single part of that rainbow is being developed by one particular raindrop.

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Blue Sky Science is a collaboration of the Wisconsin State Journal and the Morgridge Institute for Research.

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