What determines actual shape of rainbow?

02 August 2009



What determines actual shape of rainbow? I watched one the other night and it was almost flat along the horizon


Dave - Okay. The rainbow is actually always the same shape: it's always a circle.

The way a rainbow is formed is that, when the light goes into a raindrop, it kind of reflects around the back [of the raindrop] and, as it goes in and goes out, it refracts - it bends - because light goes slower in water than it does in air.

Different colours or wavelengths of light refract by slightly different amounts, so the light comes out of the raindrop at slightly different angles depending on its colour.

A raindrop is circularly symmetric, so the refraction means that you get a cone of different colours of light coming out of the drop.

So, if you look at the raindrop from certain different directions, it looks different colours. If you have a whole sky full of raindrops at different angles you see different colours depending on the direction you are looking in. You get a circular rainbow exactly opposite the Sun. If the sun is high in the sky, you just see the top of the circle and the rainbow looks flat.

If the sun is low, you can see a full semicircle; and if you are in a plane, you can see a full circular rainbow.

Chris - That makes sense. And when you see a second rainbow, that's presumably where it's gone into the raindrop bounced off the front of the raindrop, gone to the back again and come out front. So, there are two journeys and that's why you get a second rainbow around the first?

Dave - It's doing more exciting things inside the raindrop!


Many many happy returns of the day

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