How does a rainbow form?
Sunlight shines into raindrops, bends, bounces off the back of each drop and comes out split into colors. Every drop sends one color your way, and millions of drops together paint the arc. You only see it with the sun behind you and rain in front.
White light hides every color
Sunlight looks white, but it’s a mix of all the colors of the rainbow. When it hits a raindrop, it enters the water and bends, just like light through a glass prism.
Share this stepEach color bends a different amount
Violet bends the most and red the least, so the colors fan out. The light bounces off the back of the drop and comes out again, split into a spread of colors. The picture shows it simplified, as if the light passes straight through.
Share this stepThe arc comes from an angle
Each drop sends light back at about 42 degrees from the point directly opposite the sun. All the drops at that angle light up, forming a circle around that point. The ground cuts it off, so you see an arc.
Share this stepEvery raindrop is a tiny prism.
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Red is always on the outside and violet on the inside. Because the angle depends on where you stand, everyone sees their own rainbow, and you can never reach the end of it.
Common follow-up questions
Why do I only see rainbows in the morning or evening?
The sun has to be behind you and fairly low. When it’s higher than about 42 degrees, the rainbow would sit below the horizon.
What is a double rainbow?
Some light bounces twice inside the drops before coming out. That makes a fainter second arc outside the first, with the colors in reverse order.
Can you reach the end of a rainbow?
No. The rainbow moves with you, because it’s made by drops at a certain angle from your eyes. Walk toward it and other drops take over.
Is a rainbow really a full circle?
Yes. From an airplane or a high mountain you can sometimes see the whole circle. On the ground, the horizon hides the lower half.
