You’re driving after a summer storm, the sun breaks through behind you, and there it is — a perfect arc of color hung in the mist ahead. It looks close enough to touch, but you already know you’ll never reach it.
The short answer
Rainbows form when sunlight enters millions of tiny water droplets in the air, bends (refracts) on the way in, reflects off the back of each droplet, then bends again on the way out. The light exits at a precise angle — about 42° from the line between the sun and your eye — splitting into the spectrum of colors we see.
Why 42 degrees matters
Here’s where it gets interesting. White sunlight contains all colors, and each color bends at a slightly different angle when it enters water. Red light exits the droplet at roughly 42.4° from your sight line, while violet exits at about 40.6°. This precision matters: it’s the angle where light rays converge most intensely, a property named for René Descartes, who mathematically described the phenomenon in the 17th century.
When millions of droplets scatter light this way simultaneously, the ones positioned at that critical 42° angle for red light create the red band of the rainbow. The droplets slightly lower in your field of view — at 40.6° — contribute violet. The other colors (orange, yellow, green, blue, indigo) stack in between. That’s why red always appears on the outer edge of a rainbow’s arc and violet on the inner edge.
The geometry you can’t escape
Every rainbow you see is personal. If a friend stands next to you, they’re seeing light from a completely different set of water droplets — their own 42° cone of light. This is also why you can never walk to the end of a rainbow: as you move, the angle changes, and you’re always seeing light from droplets that maintain that 42° relationship between sun, droplet, and your eye.
The sun has to be behind you and fairly low in the sky (below 42° above the horizon) for this to work. Higher than that, and the geometry doesn’t allow the reflected light to reach your eyes at the right angle. Early morning and late afternoon after a rainstorm are prime rainbow time.
Why secondary rainbows are dimmer and backwards
Sometimes you’ll see a fainter, second arc above the primary rainbow, with the color order reversed — red on the inside, violet on the outside. This is a secondary rainbow, created by light that bounces twice inside each water droplet before exiting.
That extra bounce does two things. First, it flips the color order, because the light exits at a steeper angle (around 51°). Second, it scatters the light more: only about 40–50% of the light that creates a primary rainbow makes it through to form the secondary one. That’s why secondary rainbows look noticeably washed out by comparison.
Between the two arcs, you might notice the sky looks noticeably darker. This region is sometimes called Alexander’s dark band. It’s dark because the geometry of light refraction sends almost no light back toward your eye from that region — the angles simply don’t work out for that range.
The full-circle secret
Here’s the fact that surprised me when I first learned it: rainbows aren’t arcs. They’re full circles. You’re standing on the ground, so the earth blocks the bottom half. But if you’ve ever seen a rainbow from an airplane window, you might have caught the full ring. Pilots see them regularly. From the ground, the best you’ll do is a semicircle when the sun is right at the horizon.
The interesting wrinkle: moonbows and more
Rainbows don’t need sunlight — they just need bright light and water droplets in the right position. Moonbows (or lunar rainbows) appear near waterfalls or during particularly bright full moons after rain. They look almost colorless to the human eye because moonlight is much dimmer than sunlight, too faint to trigger our color-sensing cone cells. But a camera with a long exposure will reveal the colors clearly.
The size of the water droplets matters, too. Large droplets (like the heavy mist after a thunderstorm) create the brightest rainbows. Tiny droplets — fog-sized, under 50 micrometers — produce fogbows: ghostly white arcs with just a hint of red and blue on the edges, because the droplets are too small to separate colors cleanly.
FAQ
Can you ever reach the end of a rainbow?
No. A rainbow isn’t a physical object in a fixed location — it’s an optical effect created by the angle between the sun, airborne water droplets, and your eye. As you move, the rainbow “moves” because you’re seeing light from different droplets that maintain that 42° angle. There’s no endpoint to reach.
Why are some rainbows brighter than others?
Brightness depends on droplet size and density. Larger droplets and more of them mean more light gets refracted and reflected back toward you. A heavy mist after a thunderstorm with big, uniform droplets will produce a vivid rainbow. A light drizzle with tiny droplets creates a faint one.
Do rainbows always have seven colors?
The “seven colors” idea (red, orange, yellow, green, blue, indigo, violet) comes from Isaac Newton, who valued the number seven. In reality, a rainbow is a continuous spectrum — there’s no sharp line where orange becomes yellow. We see the bands our eyes and brains are wired to distinguish, but the color gradient is smooth.
Can rainbows appear in places without rain?
Yes. Any suspended water droplets will work: garden sprinklers, waterfalls, ocean spray, even the mist from a whale’s blowhole. As long as sunlight (or bright light) hits the droplets from behind you, the geometry can create a rainbow.
What causes a double rainbow?
A double rainbow appears when light reflects twice inside water droplets instead of once (the secondary rainbow). In rare cases, you can even see a third or fourth rainbow, though these are extremely faint and require very specific conditions — still air, uniform droplet size, and a dark background sky to make them visible.
The next time you spot a rainbow, take a second to appreciate the precision behind it: billions of droplets aligned at exactly the right angle, each one splitting sunlight into the same sequence of colors, all to create an arc that exists only for you. If you’re curious about other everyday light tricks, check our article on why the sky is blue.
Written for general interest and accuracy-checked, but not a substitute for specialist sources.