The physics of rainbows

Why do rainbows appear in the sky? In order to answer this question we need to be able to answer two distinct, though related, subsidiary questions:
  • Why is there an arc-shaped band of bright reflected light in the sky? This is largely a matter of geometry.
  • Why do we see a spectrum of colours within this band? This is where the physics comes in.
Once we can answer these questions it is relatively easy to deal with other topics such as:
  • Why is there sometimes a fainter secondary bow above the main one?
  • What is the order of the colours in a rainbow? In a secondary bow?
For a readable account of the basic phenomena, it is hard to improve upon the discussion on Beverly Lynds' website where, among other things, you will learn about Alexander's Dark Band (which has nothing whatever to do with ragtime). You can compare her approach with our rather colourful Raindrops' target practice.

If you would also like a hands-on demonstration, you can try our interactive Excel program. This will enable you to trace the paths of two typical rays of light (red and violet) as they are reflected and refracted within a droplet of rain. You can then experiment by adjusting the angle at which the incident ray of white light hits the raindrop.

Next: ►Target practice

Last revised October 2001

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