214 // What Shape is a Rainbow?

What Shape is a Rainbow?

Why?

We all remember being told what causes rainbows. Sunlight bounces around inside raindrops, which split the white light into its component colours. Whenever you look directly at a rainbow, the Sun will be behind you, and the rain will be falling in front of you. And to knock it on the head, the teacher showed us how a glass prism splits a ray of white light into all the colours of the rainbow.

A neat piece of misdirection, worthy of a conjurer. That explains the colours. But what about the shape?

If it's just a matter of light reflecting back from raindrops, why don't we see the colours wherever the rain is coming down? And if that were happening, wouldn't the colours fuzz out back to white, or maybe a muddy grey? Why is the rainbow a series of coloured arcs? And what shape are the arcs?

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