Understand Rainbows: Sun Behind You, Rain Ahead

How to spot one

After a shower, do not look toward the sun. Look the other way: the sun needs to be behind you, and raindrops or mist need to be in front of you. The best moments are often early or late in the day, when the sun is lower and the visible arc can look larger.

If you do not see anything at first, move a little. A rainbow is not fixed in one place like a sign in the sky. What you see depends on where you stand and which droplets send sunlight back toward your eyes at the right angle.

Why the colors separate

Sunlight looks white, but it contains many colors. Inside a raindrop, light bends as it enters, reflects from the back of the drop, then bends again as it leaves. Different colors bend by slightly different amounts, which spreads them into the familiar band.

The rainbow is really part of a circle. From the ground, you usually see only the upper arc because the horizon, buildings, or hills block the rest. From a plane or a high viewpoint, the circle can look more complete.

When there is a second rainbow

A second, outer rainbow appears when the light reflects twice inside the droplet. That extra reflection loses more light, so the secondary rainbow is fainter and easiest to see against a darker sky.

Check the color order. In the main rainbow, red is on the outside and violet is on the inside. In the second rainbow, the order is reversed. That reversal is the simple clue that you are seeing the secondary bow.

Quick rule

Sun behind you, water in front of you. One bow means one internal reflection; two bows mean an extra path with a second reflection.