Double rainbow explained

To see a rainbow is always an amazing thing that makes us stop and gaze in wonder. It is special, it is beautiful and it is not so common because everything has to be just right. For us to be blessed with an amazing arc of color there has to be rain in front of us and the sun must be behind us.

But last Wednesday, 19/11/2014, at Funchal city, this is what happened:


I was privileged to witness, not one, not a single “common” rainbow, but a wonderful double rainbow. Two rainbows one on top of the other. The first rainbow was like a shield. It looked as if Funchal city was under a protective shield like those we can see in sci-fi movies. And there was a second bigger rainbow that was not so bright and had the colors inverted (note that red is not at the top as in the first rainbow).

It was raining a lot, and just when the sun had began to shine again, at about 15:20, a perfect double rainbow formed in front of my eyes. I swear I did not start screaming like a maniac (I know it can have such effect on some people… I can tell you about it later) and I certainly did not rush to dig for gold at the end of the rainbow (this time there were 4 different places to look for it).

Double rainbow explanation

I don’t believe in pots of gold at the end of rainbows but maybe we can find some treasure trying to understand this fantastic wonder of nature.
What exactly is a double rainbow? How can a double rainbow be explained?

To understand it better let’s take a closer look to what happens inside a raindrop. The sunlight is white light. White light is made up of many colors. The raindrops are little spheres of water falling from the clouds that can act as tiny prisms. When the sunlight enters a raindrop it is refracted and is reflected at the back of the drop (at different angles depending on each color wave length). See the image:
There will be drops reflecting us the reddish color at the outermost side of the rainbow (at 42° to the sunlight), drops reflecting us the bluish colors (at 40° to the sunlight) at the inner side of the rainbow, and there will be drops in between reflecting us all the other colors. For example, we receive the image of the red color in the form of an arc because all the drops that are along that red arc are the drops that are positioned in the sky at exactly the same distance from us, in such a position that the reflected light reach us forming an angle of 42° (angle of the red light) with the sunlight . What happens to the other colors is similar only the angle being slightly different.

It means that any rainbow we see is unique. It means that the rainbow you see is your own rainbow (the rainbow is formed around you). The rainbow I see is my own rainbow. Each observer will see a different rainbow that will form around the place where the observer is. That’s the place where a set of colors converge reflected by some set of raindrops. Actually, we can say that the rainbow we see in the left eye is not the same rainbow we see in the right eye. Each eye gets a rainbow image of its own, reflected by a different set of raindrops. Isn’t that amazing?

What about the second rainbow?

The other rainbow is also a reflection of refracted colors, but is a different reflection that happens at a different angle inside the raindrops.

For the sake of simplicity let’s limit ourselves to the top of the rainbow arc. There, the sunlight can enter a raindrop and instead of being directly reflected downwards, some light can be reflected upwards. And after being reflected at the top of the drop it can end up being reflected downwards to the observer also hitting him below, but at a different angle, of around 50° (remember that the reflection angle of the first rainbow is around 40°).

This means that the drops that are sending us the second rainbow belong to a different set of drops positioned along a bigger arc. These drops make an angle between the observer and the sunlight of about 50° (52° to reddish colors and 54.5° to bluish colors). In the image below you can see how the sunlight is reflected inside the rain drops to generate the primary rainbow and the second rainbow:


Note how the colors in the second rainbow are inverted when compared to the primary rainbow.

To sum up, the double rainbow is the same atmospheric phenomenon but we are privileged with a bonus, another extra rainbow. It happens when the sun is behind the rainbow observer, generally at a low angle to the horizon. The sunlight hit the rain droplets dispersed in the atmosphere. Each droplet acts as a mini prism. The light is refracted in each of this droplets, decomposed in its different colors (according to each of their specific refractive properties) and is then reflected on the back of the rain drop. The first rainbow is caused by the first reflection in which the sunlight escapes at an angle of about 40°. But there is another reflection inside the rain droplets in which the sunlight escapes at an angle of about 50°. This second reflection is the cause of the second rainbow. So there are two very distinct rainbows, the first, primary, bright rainbow, and a second, larger rainbow which is 10° further out.

Viral double rainbow

This kind of phenomenon can make people go crazy! Don’t believe me?
A guy “went nuts” after seeing a double rainbow, screaming, crying, his video went viral:

and songs are already being dedicated to the “Double Rainbow” event:

I hope this post could shed some new light over a double rainbow. I hope you enjoyed!


UPDATE: A 60 seconds video that also explains double rainbows:

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