Science in the Sky: How rainbows and double rainbows form

The Weather Authority
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Data pix.

Time and time again, you send us your spectacular photos of rainbows and double rainbows that occur here in the Tennessee Valley. They are so bright and so vivid... But what is happening up there? What is causing them to form?

(WHNT News 19)
White light passing through a prism. The light disperses into the color spectrum. (WHNT News 19)

Rainbows occur because light passing through one medium -- like air -- bends as it passes through a different medium -- like water in a raindrop. The bending can cause the light to disperse into the different colors that you see in a rainbow, similar to what happens when light passes through a prism and splits into the color spectrum.

Sunlight as it passes through a raindrop. (WHNT News 19)
Sunlight as it passes through a raindrop. (WHNT News 19)

Often, not just one but two rainbows can be seen in the sky. Notice that the primary rainbow has the red hues on top and the violet hues on the bottom, but the double rainbow has the red hues on the bottom with the violet hues on top. What caused these double rainbows to form?

When you see a rainbow, what is happening is that sunlight is entering the raindrop and refracts, or bends. As it bends, the light diffuses into different colors, and then reflects off the back of the raindrop. It then moves to the front of the raindrop before refracting once more as it exits the raindrop and the rays enter your eyes. This is the primary rainbow that you usually see.

Sunlight passing through a raindrop. (WHNT News 19)
Sunlight passing through a raindrop. (WHNT News 19)

In a double rainbow, the light reflects not once, but twice as it moves through the raindrop. Here is the first reflection point, and over here is the second. It then refracts as it exits the raindrop.

In order to have any rainbows form, you have to have the sun behind you, and the rain in front of you. You also have to have multiple raindrops present to produce the rainbow that you see.

These are such beautiful pictures, and I am so glad that you shared them with us. If you see something beautiful like this, be sure to send them in by emailing them to In the meantime, I hope you have a wonderful -- and colorful -- week of weather where you live.

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