Show Details

Round Rainbows

March 15, 2007

A listener asks: Why are rainbows round?


How the rainbow gets its shape. I’m Bob Hirshon and this is Science Update.

Thanks to our Web site, we got a perplexing question from Prazsák István in Hungary. He wants to know why rainbows are round. Well, physicist Robert Greenler of the University of Wisconsin-Milwaukee points out that you can only see a rainbow if the water droplets in it reflect sunlight right into your eye. Droplets that can do that happen to be 42 degrees from your shadow. And if you pick out all the droplets in the sky that are 42 degrees from your shadow in all directions, they’ll be in a circle—and that, Greenler says, is the rainbow.

ROBERT GREENLER (University of Wisconsin-Milwaukee):
Now usually from the ground you can only see part of this. If you get in an airplane you have the possibility of seeing the complete circle, and in fact photographs have been taken of that.

You, too, can submit a science question by going to If we use it on the air, you’ll win a mug. I’m Bob Hirshon, for AAAS, the science society.