What makes a rainbow?
A rainbow forms when sunlight enters raindrops, bends, reflects off the back of each drop and splits into its colors on the way out, like light through a prism.
Sunlight looks white, but it is a mix of colors with different wavelengths. When a beam enters a raindrop, it slows down and bends, a process called refraction. Each color bends by a slightly different amount, so the light spreads out. The light then reflects off the back of the drop and bends again as it leaves.
Red light leaves the drops at an angle of about 42 degrees from the direction of the sunlight, and violet at about 40 degrees. Millions of drops send different colors toward your eyes, building the familiar arc of red, orange, yellow, green, blue, indigo and violet, with red on the outside.
When you can see one
You need the Sun behind you and rain in front of you, and the Sun must be fairly low in the sky. That is why rainbows often appear in the early morning or late afternoon after a shower. A fainter second rainbow, with the colors reversed, appears when light reflects twice inside the drops.
Did You Know?
A rainbow is actually a full circle. From the ground we only see the top part, because the Earth gets in the way, but pilots and mountaineers sometimes see the complete ring. No two people see exactly the same rainbow, since each viewer gets light from different raindrops.