A rainbow is one of the few weather sights that makes nearly everyone stop and look up. It appears as if by magic after a passing shower, arching across the sky in perfect bands of color. But there's nothing magical about it — a rainbow is a beautifully precise piece of physics, created by sunlight, raindrops, and the exact angle between you and the sun. Once you understand the recipe, you can even predict when and where to look for one.
The whole show comes down to what happens to a beam of sunlight when it enters a single drop of water. Multiply that by millions of drops, and you get the arc we all recognize.
Sunlight looks white, but it's actually a blend of every color mixed together. When a ray of sunlight strikes a raindrop, three things happen in quick succession:
1. Refraction: the light bends as it passes from air into the denser water, and because each color bends by a slightly different amount, the white light begins to fan out into its separate colors. 2. Reflection: the light bounces off the back inside surface of the drop and heads back toward you. 3. Refraction again: as the light exits the drop into the air, it bends a second time, spreading the colors even further apart. The result is that each drop sends a tiny spectrum of color back to your eyes.
Here's the key fact that explains almost everything about rainbows: the light leaves each drop at a very specific angle — about 42 degrees from the point directly opposite the sun. Every raindrop sitting at that 42-degree angle from your line of sight sends color to your eye, and all those points together trace out a circle. From the ground you usually only see the top half, so it looks like an arch. From an airplane, you can sometimes see the full circle.
| Ingredient | Why it's needed |
|---|---|
| Sunlight | Provides the white light that splits into colors |
| Raindrops (or mist) | Bend and reflect the light back to you |
| Sun behind you | Light must come from behind to reflect forward |
| Low sun angle | Sun below ~42° in the sky, so morning or late afternoon |
The colors run red, orange, yellow, green, blue, indigo, violet — from the outer edge of the arc to the inner edge. They always appear in that order because each color bends by a fixed amount: red light bends the least and ends up on the outside, while violet bends the most and lands on the inside. The order never changes because the physics never changes.
Sometimes a fainter second arc appears above the main one, with its colors reversed — red on the bottom instead of the top. This double rainbow happens when light reflects twice inside the raindrops before leaving. The extra bounce flips the color order and loses some brightness, which is why the secondary bow is always dimmer.
You may also notice the sky looks brighter inside the main arc than outside it. That darker band between a double rainbow's two arcs has a name: Alexander's band. Rainbows can also form in any water spray — a garden hose, a waterfall, or sea mist — as long as the sun is positioned correctly behind you.
Your best chances come in the early morning or late afternoon, when the sun sits low in the sky, right after a rain shower has passed and the sun breaks through behind you. Scattered showers with sunny breaks — the classic mix you'll often see in an hourly forecast — are ideal rainbow weather. Keep an eye out as storm clouds drift away and sunlight returns.
A rainbow forms when sunlight refracts, reflects, and refracts again inside countless raindrops, splitting white light into its colors and sending them back to your eyes at a precise 42-degree angle. That fixed angle is why rainbows are curved, why the colors always line up the same way, and why you can never quite reach the end. Next time a shower clears with the sun at your back, turn around and look — the physics is waiting.
→ Catch the next sun-and-shower mix on ClearCast