How are rainbows formed?

The longer answer

Light changes direction a little when it passes from air into water, the same reason a straw looks bent in a glass. Inside a raindrop, the light bends on the way in, bounces off the back of the drop, and bends again on the way out.

Every color bends by its own amount. Red bends the least and violet bends the most, so they leave the drop at slightly different angles. That is why a rainbow always has the same order: red, orange, yellow, green, blue and violet.

Because the light bounces back toward you, the sun has to be behind you. Turn to face the sun and the rainbow disappears. It is also why no two people ever see exactly the same rainbow: each of us sees light from a different set of drops.

Three things to know

• A rainbow always has its colors in the same order. Red is always on the outside of the arc and violet on the inside, because red bends the least as it passes through a water drop.
• Soap bubbles show rainbows too. A bubble's skin is so thin that light bouncing off its outside and inside mixes together, pulling the colors apart.
• You can only see a rainbow with the sun behind you. The light bounces back out of the drops toward your eyes, so the drops have to be in front of you.

Try it at home: Make a rainbow in the yard

1. Pick a sunny morning or late afternoon, when the sun is low.
2. Stand with your back to the sun.
3. Spray a fine mist from a hose in front of you.
4. Move the spray up and down until the colors appear, then check the order: is red on the outside?

No hose? Blow soap bubbles in the sunshine and watch the colors swirl on their skins.