How does a rainbow form?

After the rain, with the Sun peeking through the clouds, an arc of colors sometimes appears across the sky. A rainbow seems magical, but it isn’t an object that’s “out there”: it’s sunlight playing with millions of water droplets. And behind its beauty is pure physics of light. Let’s discover it.

White light hides all the colors

The starting point is surprising: sunlight looks white, but it contains all the colors of the rainbow mixed together. Isaac Newton proved this by passing a beam of light through a glass prism: the light came out split into a band of colors.

A raindrop does exactly what that prism does. That’s the key to the whole phenomenon.

What happens inside each drop

When a ray of sun enters a raindrop, three things happen in an instant, as NOAA SciJinks explains:

1. It refracts (bends) as it enters the drop, because light changes speed when passing from air to water.
2. It reflects off the inner wall of the drop, bouncing back.
3. It refracts again as it leaves the drop toward your eyes.

On that path, each color bends at a slightly different angle: red a bit less, violet a bit more. So the white light spreads out like a fan into the seven classic colors: red, orange, yellow, green, blue, indigo, and violet.

Why it’s shaped like an arc

Here’s one of the most curious details. Each drop returns the light at a very specific angle (around 42 degrees relative to the direction of sunlight). Only the drops that are exactly at that angle from your eyes send you color.

The set of all those points that meet that condition forms a circle (or rather, the part of a circle you see: an arc). That’s why the rainbow has that characteristic curved shape. It’s actually circular; we only see the upper half because the ground cuts off the other half. From an airplane, you can sometimes see the complete, round rainbow.

Why it “moves” with you

Another oddity: the rainbow isn’t in a fixed spot in the sky. It depends on where you are. Since the angle is measured from your eyes, each person sees “their own” rainbow, formed by different drops. If you move, the arc moves with you, and that’s why it’s impossible to reach “its end”: it always moves away, because it’s not a physical object, but a personal optical effect.

Double rainbows and other cousins

Sometimes a second arc appears, fainter and above the main one, with the colors reversed. It’s a double rainbow, and it happens when light reflects twice inside the drops before leaving. There are also similar phenomena, like the halos around the Sun or Moon, caused by ice crystals instead of water drops.

Physics that moves us

A rainbow is a beautiful reminder of how nature works: water, light, and precise angles combining to paint the sky. Knowing how it forms —light refracting and reflecting in each drop, the 42-degree angle, your own position— doesn’t take away an ounce of its magic; on the contrary, it makes you look at the next one with more wonder. Because that arc of colors is, in part, being created by you as you look at it.