Last week after a particularly good thunderstorm I looked out the front door to find one of my favorite things, a full double rainbow. Depending on where you live, rainbows can be a rare thing. I am lucky enough to live in a place where afternoon thunderstorms are common in the summer and clear out by sunset, which equals the perfect recipe for some pretty spectacular rainbows.
But how do they work? To start, we perceive sunlight as white light, when really a rainbow is hidden in every ray. Red, orange, yellow, green, blue, indigo and violet all combine to create "white" light. So what happens when this white light passes through a refracting object like a prism or a rain drop? The sunlight gets dispersed, revealing its beautiful color spectrum. The rain drop also does another wonderful thing, it refracts the light in such a way that it bends, creating the traditional rainbow arc. What you might not know is that if the earths horizon was not there, you would see an entire circle of rainbow! Sometimes you can actually see this if you view a rainbow from up in the air.
So how do you get a rainbow? A few factors come into play. For one you have to have the sun behind you at a lower level, and two you need to have rain in front of you. When these two elements combine, rainbow magic happens. And when conditions are just right you get a full rainbow, or better yet a double rainbow like I saw last week.
Double rainbows aren't actually a second rainbow, but rather a reflection of the lower, original rainbow. If you look closely you can see that the color order of the outer rainbow is reversed. Another interesting occurrence ia a set of color bands under the original rainbow called a supernumerary rainbow. These consist of pink, green and purple bands of color that are a result of interference of light waves on the raindrops.
The physics of light and color are endlessly entertaining and thankfully we have wonderful examples like rainbows to illustrate how they work. Thank you also to Rene Descartes, who was the first, in 1637 to make the connection between round water droplets and the interaction of refracting light. What other natural phenomenon could so skillfully bring together science and history than the beautiful rainbow?
All this knowledge though and I still wonder... where is that pot of gold?
* Last time in Nature: Back Yard Races