Stand in the right spot after a storm, with the sun at your back and rain still falling ahead of you, and you’ll see it: a perfect, glowing arc stretching across the sky. But why are rainbows curved rather than straight, or shaped like something else entirely? The answer comes down to simple geometry — every rainbow is really a circle, and the arc is just the part of that circle you happen to be able to see.

Light Bending Inside a Raindrop

A rainbow starts when sunlight hits a raindrop and refracts, or bends, as it passes from air into water. That light then reflects off the inside back wall of the droplet and refracts again on the way back out, splitting into its component colors in the process. This happens inside millions of raindrops at once, all lit by the same sun, which is why the effect appears as a broad band rather than a single point of light.

The 42-Degree Rule That Shapes the Arc

Here is the key detail: that bending and bouncing inside a raindrop sends light back out at a very specific angle — about 42 degrees from the direction the original sunlight came from, according to physicist Robert Greenler of the University of Wisconsin-Milwaukee. Only raindrops sitting at that precise angle relative to your eye and the sun send you rainbow-colored light; every other raindrop just looks like a normal drop. Because that 42-degree condition holds true in every direction, not just one, it traces out a full circle around a spot in the sky called the antisolar point, directly opposite the sun from where you’re standing.

Why We See an Arc Instead of a Full Circle

If rainbows are really circles, why does everyone see a curved arch instead of a full ring? The answer is the ground gets in the way. The antisolar point sits exactly opposite the sun, which usually puts it below the horizon when the sun is above it — so the bottom portion of the circle would have to shine up through the earth, according to National Geographic Education. Only the upper part of the circle, arcing above the horizon, is visible from the ground. The lower the sun sits in the sky, the higher and more complete the visible arc becomes, which is why rainbows often look biggest and most dramatic in the early morning or late afternoon. From a high enough vantage point, like an airplane, observers can sometimes see the full circle with no horizon to cut it off.

Double Rainbows and Reversed Colors

Sometimes a fainter second arc appears outside the primary rainbow. That secondary rainbow forms when light reflects twice inside the raindrop instead of once, exiting at a wider angle of about 51 degrees. The extra bounce also flips the color order, so red sits on the inside of the primary rainbow but on the outside of the secondary one — a detail that is easy to spot once you know to look for it.

Every Rainbow Is Personal

Because a rainbow’s position depends entirely on the angle between the sun, the raindrops, and your own eyes, no two people ever see exactly the same one. Step a few feet to the side and the antisolar point moves with you, shifting which raindrops are sending light your way. It is a small, strange consequence of the geometry involved — the rainbow you photograph is, in the most literal sense, made just for wherever you happened to be standing.