How does a rainbow form?

After a storm, when the Sun peeks out from behind the clouds, a beautiful arc of colors sometimes appears in the sky. It is a rainbow, and to see one you need a mix of rain and sunlight.

The ingredients of a rainbow

To get a rainbow you need water drops in the air, a shining Sun and you standing with the Sun behind your back, looking toward the rain. That is why it always appears on the side of the sky opposite the Sun.

White light contains every color

Sunlight looks white, but it is made of many colors mixed together. Each color has a different wavelength, and that makes them bend differently when light passes from one material to another, such as from air to water.

What happens inside the drop

When a ray of sunlight enters a raindrop, it bends: this is refraction. Inside the drop it bounces off the back wall, like a mirror, and bends again as it leaves. Because each color bends a little differently, white light fans out into a spread of colors. Millions of drops do the same at once and form the arc we see.

The seven colors

From the outside in, a rainbow has red, orange, yellow, green, blue, indigo and violet. In fact there are many shades in between, and splitting it into seven colors is mostly a habit.

Why is it shaped like an arch?

It is really a full circle. From the ground we only see the part above the horizon, but from a plane or a high mountain you can sometimes see the whole circle.

The double rainbow

Sometimes a second, fainter arc appears above the first one, with the colors in reverse order. It happens because light bounces twice inside the drops.

Experiment: your own rainbow

Place a glass of water on the edge of a table next to a sunny window, and put a sheet of white paper on the floor where the sunlight passing through the glass lands. You will see a band of colors on the paper. You can also make one with a garden hose, with the Sun behind you, on a sunny day.