Double the positivity: How are double rainbows formed?

Double rainbows are two arcs of colour decorating the sky – literally two for the price of one. The main rainbow is usually brighter and of the usual pattern (red on top, violet at the bottom), and the second rainbow — which is called the secondary arc — is fainter, and the colours are in reverse order. Now how do these rare beauties occur? Asked and answered.

The basics

Before we get into the formation of a double rainbow, let’s see how a single one is formed. Sunlight is made of many different wavelengths, or colours, that combine as one to make white light.

When sunlight travels through a rainbow, it slows down due to change in pressure from air to water. This pressure further causes the light to split into seven different colours, each of a different wavelength. This process is called refraction. The longest wavelength is in red and the shortest is… you guessed it – violet!

Breaking the formation down

Double rainbows are formed when the sunlight reflects twice inside the raindrop. The first time it reflects leads to the primary arc. Whatever light is left refracts at a different angle, forming into a second rainbow, which is usually fainter and the colours are reversed.

All rainbows are double rainbows

Theoretically speaking, all rainbows are double rainbows, but since the secondary bow is always fainter than the primary, it may be too weak to spot in practice. Or in some cases, weather conditions may not be right. So, what the eye is truly seeing is only half a phenomenon.

A double rainbow is seen during a heavy rainfall over Djurgarden and Nacka in Stockholm, Sweden.
| Photo Credit:
AP

Double rainbows aren’t just beautiful — they’re a showcase of light physics in action. So, the next time you see one, note this — you’re not just going to have a great day ahead, you’re witnessing science create something truly stunning.