What is the multicolored arc seen in the sky after rain?

The multicolored arc seen in the sky after rain is a rainbow. It forms when sunlight enters raindrops, bends as it passes from air into water, reflects from the back of the drop, and bends again as it exits.

Refraction separates white sunlight into its component colors because different wavelengths bend by slightly different amounts. Internal reflection then sends the separated light toward an observer. A primary rainbow is centered opposite the Sun, which is why the Sun must generally be behind the viewer and rain must be ahead.

The familiar primary arc displays red on its outer edge and violet on its inner edge. Each individual raindrop sends only a small amount of colored light toward the observer, but millions of drops at the right angles combine to produce the visible bow. From high elevations, a rainbow can sometimes appear as a complete circle rather than a partial arc.

A secondary rainbow may appear outside the primary one when light reflects twice inside raindrops. Its colors are reversed and it is usually fainter. An aurora, sundog, or halo can also be colorful, but each is produced by different atmospheric optics rather than ordinary raindrops.