How does rainbow occur

When light is reflected, refracted and scattered in a drop of water, a rainbow appears in the sky. In this new lesson of the Windy.app Meteorological Textbook (WMT) and newsletter for better weather forecasting you will learn more about what rainbow is and how it is formed.

How light turns into a rainbow

Light consists of particles that behave like waves. These waves are different in length, and light of different wavelengths is perceived by our eyes as different colors. In sunlight particles of different waves overlap creating white light.

But when white light passes from one medium to another — from air to a drop of water, for example — it refracts. Particles of different wavelengths refract with different extent, so they «leave» the drop in different places. White light as a result breaks up into different colors — from purple to red, which we call the colors of the rainbow.

Thus, a rainbow is sunlight behind your back which «broke» up on drops of rain in front of you and reflected into your eyes. So to see a rainbow you always have to stand with your back to the sun.

White light breaks up into differrent colors. Illustration: Valerya Milovanova / Windy.app

Why is the rainbow round

In the picture above you could notice the number 42. This is the answer to why a rainbow is shaped like an arc, semicircle or circle. No, we’re not kidding.

In fact, in a drop of water light is mostly reflected at an angle of 42 degrees. If you imagine how light, given the shape of the Sun, will be reflected somewhere at an angle of 42 degrees, you get the shape of a rainbow.

Reflection of light in a drop of water. Illustration: Valerya Milovanova / Windy.app

Because of this, the position of a rainbow is unique for each person watching it — although it can be seen and photographed from different places, only you see the particles reflected into your eyes.

It also explains why you can’t catch up with the rainbow — it will just move with you, as there will be other particles reflected by the droplets a little further.

Unusual kinds of rainbows

If you get high above the ground — for example, on a plane — the ground will no longer block part of the light reflected from the droplets, so the rainbow will become round. At a certain angle light may be reflected by the droplets twice before it gets to your eyes, and then you can see a double rainbow.

It is important that sunlight is not necessary for a rainbow — it’s enough to have a strong light source behind your back and very humid air in front. Because of this it’s possible to see a lunar rainbow — a moonbow — though it’s very rare.

Nonparallel double rainbows or reflection rainbows are even more rare — the second rainbow in this case is born from light reflected from the surface of a body of water.