How Rainbows Form

The shower has passed, but the air still glitters with raindrops. The Sun breaks through behind you, and suddenly there’s a ribbon of colour across the sky. How did water and sunlight make that happen?

Picture a single raindrop. Sunlight enters it and bends as it passes from air into water. That bending is called refraction. Sunlight looks white, but it contains a continuous range of colours. Each colour bends by a slightly different amount, so the light begins to spread out. Some of it reflects off the inside of the drop, then bends once more as it leaves. If that light travels toward your eyes, you see colour.

One drop isn’t painting the whole bow. Countless drops send light toward you, but only drops in particular directions send each colour to your eyes. The bright red edge of an ordinary rainbow appears about forty-two degrees from the point in the sky directly opposite the Sun; violet lies toward the inside of the arc. Those directions trace part of a circle. The ground usually hides the rest. That’s why you look away from the Sun to find a rainbow—and why someone standing beside you receives light from a slightly different set of drops.

Sometimes there’s a second, fainter arc outside the first. Its light has reflected twice inside the drops instead of once, and its colours run in reverse order. So if you spot a double rainbow, you’re seeing two different routes through the rain.

Want to look for one? Wait for sunlight with rain—or a fine spray of water—in front of you, and put the Sun behind you. The rainbow isn’t an object waiting at the end of a field. It’s what happens when light, water, and your particular viewpoint line up.