Topics Physics and everyday science

Why do we see rainbows?

We see rainbows because sunlight enters countless raindrops, bends, bounces off the back of each drop, and bends again on the way out. Because each color bends by a slightly different amount, white light spreads into a band of colors. You see the result only when the sun is behind you and rain is in front of you.

What makes it interesting is that a rainbow is not a thing hanging in the sky. It has no fixed location. Every person sees their own rainbow, built from different drops, and if you walk, it seems to walk with you. The arc is a circle, and the ground usually cuts it off. From an airplane you can sometimes see the full circle. There is also the fainter second bow, with its colors reversed, and the dark band between the two.

An episode would walk through the path of one ray of light, explain why the angle is always about the same, and why the colors come in that order. It would also cover the extra bows and what stays uncertain about how we name colors. bre is an app whose hosts are AI, so they can be wrong, but you can press Talk and ask them a question about any step.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. Light is a mix of colorsSunlight looks white, but it contains all the colors of the spectrum. A prism shows this, and a raindrop acts a lot like a tiny prism.
  2. What happens inside one raindropLight slows down entering the water and bends, reflects off the back inside surface, and bends again leaving. Each color bends slightly differently, which fans them apart.
  3. Why the angle is always about 42 degreesRed light comes back strongest at roughly 42 degrees from the line pointing away from the sun. That fixed angle is why the bow always forms an arc of the same size.
  4. Why your rainbow is yours aloneDifferent drops send color to different eyes, so no two people see the same rainbow. It also explains why the bow seems to move when you do.
  5. Why it is an arc, and sometimes a circleThe rainbow is really a cone of light around the point opposite the sun. The ground hides the lower part, but from a high place the full circle can appear.
  6. Double rainbows and the dark bandLight that reflects twice inside drops makes a fainter second bow with reversed colors. Between them is a darker strip of sky.
  7. Counting colors is partly a choiceThe spectrum changes smoothly, with no sharp edges. The usual list of seven colors is a convention, and cultures divide it differently.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Cal. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Here's a question I've never fully answered. When you point at a rainbow, what exactly are you pointing at? Because it isn't an object sitting over there.

  2. CalAI host

    Right, that's the weird part. I always pictured a big colored arch hanging in the sky, like a bridge. But if I'm told it isn't there, then what is it?

  3. breAI host

    It's light, arriving at your eyes from a very particular set of raindrops. Different drops, different angle, and someone standing next to you gets a different rainbow.

  4. CalAI host

    Wait. So my rainbow and yours are different rainbows? Hold on, how does that actually work?

  5. breAI host

    Start with one drop. Sunlight goes in, bends, bounces off the back wall, and comes out bent again. The bending is the trick, because red and violet bend by slightly different amounts.

  6. CalAI host

    So the drop is basically a tiny prism with a mirror stuck on the back. Honestly, that makes me feel better about all those prism posters.

  7. breAI host

    Pretty much. And the strange part is the angle. Out of all the directions light could leave, one angle shows up strongest, and that decides where the rainbow sits.

  8. CalAI host

    Okay, so that's why it always shows up opposite the sun. I want to know why it's always the same size, too.

Questions people also ask

Why do rainbows appear after rain?
A rainbow needs two things at once: sunlight and water drops in the air. After a shower, drops are still floating while the sun comes out. You also need the sun behind you and the rain in front, so the light can return to your eyes.
Why are the colors always in the same order?
Each color bends by a slightly different amount when it passes through water. Red bends least and violet most, so red comes out at a slightly larger angle. That puts red on the outside of the main bow and violet on the inside, every time.
Can you ever reach the end of a rainbow?
No. A rainbow is light arriving from a specific angle, not an object at a fixed place. When you move, the drops that send light to your eyes change, so the bow appears to move with you. There is no end to reach.
Why is the second rainbow reversed?
The second bow comes from light reflecting twice inside each raindrop instead of once. The extra reflection flips the order of colors, so red is on the inside. It is also fainter, because some light is lost at each reflection.

Related topics

More: all 300 topics, physics and everyday science, or the longer reads on /learn.

bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.