We all know the feeling, rain lashing down on your face, soggy socks and a long walk ahead to get home, but suddenly the sun comes out. You look around, and you see one of nature’s beauties, the rainbow. Now, the pot of gold at the end might be a terrible lie, but judging by the Youtube sensation Double Rainbow Guy, these are worth a second look at for their magical, and perhaps even intoxicating beauty.
Rainbows are the product of refraction, which is the way that the light changes when the sunlight hits the raindrops that are headed for the heads of other poor people who also left their umbrellas at home. We can model this with a simple triangular prism in a laboratory set up with a light box.
Now, here we see a whole rainbow coming out of one beam of light, which shows us how white light is made up of all the colours of the rainbow. But that doesn’t quite look like the rainbow in the sky does it? It’s not made up of lots of little tiny rainbows all next to each other, so how can we see one big rainbow instead.
So when we look at this like a huge stack of raindrops (triangles) with the sun streaming in from one direction, you can see that even though each raindrop is creating all the colours of the rainbow, where your eye is you can only see one of the colours. And because all the triangles are the same, and the rules of refraction stay the same, we see big strips of colour that relate to the big lines that head into your eye in the picture.
Interested? You can go into the maths a little more with my next post, behind the scenes and into the world of refraction.