Iris, in her thousand hues enrobed traced through the sky her arching bow.

– Ovid, Metamorphoses 11. 585 ff, (trans. Melville) (Roman epic C1st B.C. to C1st A.D.)

On the afternoon of Sunday, June 22nd, a passing shower produced this stunning double rainbow in Chicago’s eastern sky. You can see a larger version of this picture here.

Rainbows are produced by a combination of refraction and reflection as sunlight encounters raindrops. Some of the sunlight passing through the upper part of a drop is refracted into its constituent spectral colors, then reflected off the back of the drop, and refracted again as it passes back through the front of the drop and toward the observer.

A secondary rainbow is occasionally visible, caused by light striking the lower parts of drops and reflecting twice inside each drop before exiting. That second reflection is what makes the secondary rainbow appear with its colors reversed: red at the bottom and blue at the top.

René Descartes, the French polymath, derived the geometric relationships among sun, observer, and the droplets responsible for primary and secondary rainbows in his treatise of 1637, “Discourse on Method.” Here’s his sketch of it: