Creating rainbows

Q: What causes rainbows after it stops raining?
Brandon, 8 years old, Albuquerque, NM

A: Light and water create rainbows. However, to see a rainbow, it must be raining in one part of the sky, the sun shining from the opposite part, and you must have your back to the sun. Then raindrops, like tiny prisms, scatter the white sunlight into brilliant colors--red, orange, yellow, green, blue, indigo, violet--and make a rainbow.

The drops bend the light to scatter it. Light slows when it goes through water and this causes its path to bend.

A skater experiences the same thing, when suddenly a skate hits a rough spot and that skate slows. The skater turns toward the slower skate.

Light bends in a similar way both when it enters and leaves the raindrop. Click for Figure 2.

Some colors slow more than others and therefore bend more. Violet light, for example, travels slower than red. That separates the violet from red light.

The red light emerges at a 42-degree angle to the original white ray. The violet reappears at only 40 degrees. The raindrop has separated the white light into colors and then re-directed the colored light back towards you. That's why you see the rainbow.

Now suppose the raindrop that we have been discussing is directly in front of you and at an angle of 42 degrees up: the angle of red light. Click for Figure 3. Then the red light from that drop hits your eye and you see red. The violet light from that drop, however, is higher (at 40 degrees) and misses your eye. Maybe it hits your hat and you don't see violet. How, then, do you see all the colors in the rainbow? And how do you see the whole bow and not just a single light?

In Figure 3, you are looking straight in front at 42 degrees to see the red light scattered by our raindrop but remember, there are lots of raindrops in the sky. Look to your right or left, but still at 42 degrees. You will see more red. In fact, you will see an arc of red all coming into your eye at 42 degrees.

Now look straight in front but up a little, at 40 degrees: you see violet from other drops. Again sweeping your eye to the right or left but at 40 degrees shows a bow of violet.

That's how all the colors form a grand bow stretching from horizon to horizon.

Further Surfing:

Look down on a rainbow to see a whole circle, WeatherQuesting

Why the second rainbow colors are backwards, WeatherQuesting

What a rainbow looks like to a dinosaur, WonderQuest

Glory (circle) rainbows seen from a plane, WeatherQuesting

Why the inside of a rainbow is bright, WeatherQuesting

Unidata: What is a rainbow?

(Answered Sep. 20, 2002, updated July 20, 2007)