Light and raindrops work together to create a ring of colored light opposite the sun.
Gallery: Rainbows around the world
We see part of that ring as the curved arc of a rainbow. Here’s how it works: the sunlight that shines into a raindrop leaves that raindrop at an angle of 138 degrees from the path that the light traveled before it entered the drop. That’s the “rainbow angle,” discovered by Descartes in the year 1637.
If the light left at 180 degrees, it’d head straight back toward the sun. As it is, the light is traveling in a direction somewhat back toward the sun, which is why you always see rainbows when the sun is behind you.
The sunlight emerges from many raindrops at once. The combined effect is a mosaic of light, spread out in an arc in the sky. But that’s not the end of the story. When sky conditions and your vantage point are perfect, the rain and sun work together in this way to create a complete ring of light – a circle rainbow. You’ll never see this from Earth’s surface because your horizon gets in the way.