Why double rainbow is so rare?

Rainbows are difficult to witness but easy to create a moment. 

The rainbow comes from the Latin word that means rainy arch. In times of the Greeks and Romans, rainbows were a path to link humans to immortals. Rainbows are one of the most fascinating and beautiful phenomena of Mother Nature. Similarly, you have to be lucky enough to witness a double rainbow in person.

But, what is a double rainbow?

A Double rainbow is a colourful display in the sky where we see two spectacular bands. This phenomenon mainly occurs at the time when the sun is low, either at the time of sunrise or sunset. The secondary rainbow is more pastel than bright in tone as compared to the primary one.

Both rainbows disperse over a wide area in the sky, but the colour sequence varies. The primary band follows VIBGYOR, whereas the secondary rainbow appears as ROYGBIV.

What is the science behind the formation of Double Rainbows?

We are aware that the formation of a rainbow is due to two phenomenon- reflection and refraction. After the sunlight enters the droplet, it reflects, and when it moves from the air to the water droplets and back into the air again, each colour of lights with visible spectrum refracts at different angles. This scattering of light creates a rainbow.

Similarly, when the process of total internal reflection takes off, sunlight bounces around inside the water droplets twice. It is how a secondary rainbow, exactly nine degrees above the primary, is created. Since few of the lights reflect during the formation of the second rainbow, it appears to be fainter. It also results in reversing the order of colours of the spectrum.

Alexander’s band is a dark area between the two bows. Alexander of Aphrodisias noticed this for the first time about 1800 years ago. This area appears darker because the scattered lights by the water droplets in that range do not reach our eyes.

Are double rainbows rare?

Not really! Sometimes double rainbows formed are not captured by human eyes due to the un-clear sky. But in the world of rainbows, we do have rare varieties. 

Supernumerary rainbows This happens when there is an interference of overlapping light waves. Here, you can see an extra repeated band of colour inside of the primary rainbow.

Twinned Rainbows These are different from double rainbows as they are non-inverted rainbows that split up from the same base. According to some theory, two different sizes of raindrops in a storm cause twinned rainbows.

So next time you see a double rainbow, feel free to consider yourself lucky.

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