Snell’s Law

When sunlight hits a raindrop, some photons glance off the surface. But others penetrate, bounce off the opposite side of the droplet, and shoot back out the front. That internally reflected light is what makes rainbows, and their distribution of colors is determined by Snell’s law, which describes how light refracts as its speed changes. It works like this: Light moves slower in water than in air, and the sudden deceleration alters its direction of travel. Short wavelengths like violet bend more than long ones like red, causing that beam of white sunlight to fan out into a spectrum of colors. The result: Each color hits the back of the raindrop at a different spot and bounces out at a slightly different angle. Now picture a sky full of raindrops. Each one reflects the full spectrum, but because of the varying angles, we see different colors from drops at different heights. Imagine a line extending from your head to its shadow on the ground. The red light that you see is from drops that are 42.4 degrees above that line. The violet band, at the bottom of the rainbow, is from drops that are a little lower—40.7 degrees.

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2 Responses to Snell’s Law

1. Phil Krause says:

You can also get a double rainbow where a fainter, second rainbow appears out side of the first with its colours reversed. A normal rainbow is caused by one reflection inside the raindrop and two refractions, one as the light enters and the other as it exits. The double rainbow has two refractions and two reflections inside the raindrop so less light comes towards you and it appears fainter. Instead of 42 degrees, the secondary rainbow is between 50 and 52 degrees.

Even more rarely, you can see an inverted rainbow that looks like a giant smile and is higher in the sky. I believe these to be formed inside ice crystals rather than raindrops. The one I saw didn’t look as colourful as a normal rainbow and was higher in the sky. If you could have seen the entire circle it would have been contained in the sky. The sky was darker above this rainbow rather than below it with a normal one.

2. Deskarati says:

Great comment Phil, It has inspired me. I feel a post coming on!