A rainbow looks like a band of color hanging in the sky. But it is not a thing in one place. It is just light that reaches your eyes only when you stand in the right spot. To see one, you need bright sunlight and water drops in the air. You also need the sun behind you, so the drops are in front of you.

Sunlight looks white. But it is made of all the colors of the rainbow mixed together. A prism is a piece of glass that splits a beam of sunlight into rainbow colors. The colors that come out of a prism and the colors of a rainbow are the same, in the same order. Red is on the outside, and blue and violet are on the inside.

Here is the main idea. Light bends when it goes from air into water. This is called refraction. Refraction is the bending of a wave as it passes from one material to another. One example is light going from air to water. Light moves more slowly in water than in air, which is why it bends when it enters a raindrop.

Each raindrop acts a little like a tiny prism, splitting sunlight into its colors. A rainbow shows up opposite the sun, so you must look away from the sun to see one.

Rainbows often show up after a rain shower, when the sun comes out. The best chances to see a rainbow are in the early morning or the late afternoon and evening. The sun is low then. Sometimes you can see your own shadow while it rains. If the sun is low enough, the rainbow circles around the shadow of your head. A rainbow shows up best against a dark storm cloud.

You can see a rainbow in the spray from a waterfall, a garden hose or a lawn sprinkler. Stand with the sun at your back and the spray in front of you.

Sometimes two rainbows show at once. There is a bright one, and a fainter one above it with its colors flipped. A double rainbow happens when light bounces twice inside the raindrops instead of once.

Rainbows do not need rain. Fogbows are pale, nearly white rainbows that form in fog, mist or cloud.

People usually name six or seven colors in a rainbow. The classic order is red, orange, yellow, green, blue, indigo and violet. People remember it as ROYGBIV.

A rainbow is really a full circle. The ground usually hides the lower part. So we see only an arc. From a plane you can sometimes see a whole circle rainbow.

Words to know

• refraction: light bending
• prism: color-splitting glass
• fogbows: pale fog rainbow
• arc: curved line

The short version

A rainbow is light that water drops in the air split into colors. It reaches your eyes only when the sun is behind you and the drops are in front of you. Each raindrop bends sunlight, and acts a little like a tiny prism.

Quick facts

• Sunlight looks white, but it is made of all the colors of the rainbow mixed together.
• Light bends when it goes from air into water. This bending is called refraction.
• A double rainbow happens when light bounces twice inside the raindrops instead of once.
• Fogbows are pale, nearly white rainbows that form in fog, mist or cloud.
• The best chances to see a rainbow are in the early morning or the late afternoon and evening, when the sun is low.