Mr Newton's Rainbow

I'm currently reading for review a book called Quantum Physics for Poets (the next step, I suppose, from How to Teach Physics to your Dog). In it, the authors comment

A glass prism hanging in our window splits the white sunlight into its spectral constituents Red-Orange-Yellow-Green-Blue-Indigo-Violet (ROY G. BIV)

Now, leaving aside the rather bizarre idea that 'Roy G. Biv' is somehow a useful way of remembering anything, I thought it rather sad that this book, written by a Nobel laureate and friend, passes on as wisdom without comment the idea that there are seven colours in the rainbow. It's a load of tosh, for which we have to thank Isaac Newton.

If you take a look at a rainbow and look for blocks of colour, it's hard to see more than six. Alternatively, if you consider the rainbow of colours on your computer screen, it is likely to be made up of millions of subtly different hues. Either way you consider it, seven is wrong.

There's a good reason for this. There was no scientific basis for Newton's assertion that there are seven colours. We aren't absolutely certain, but the best supported theory for why he came up with this number is because there are seven musical notes - A to G - before you come back to the A in the octave. If music had seven notes, Newton seems to have argued, a rainbow should have seven colours, and he came up with a set to match.

Interestingly, he was lucky to be able to come up with those particular colours. One of Roy G. Biv's constituents didn't exist a few decades earlier. When I do talks on this subject and ask people to guess which colour didn't exist they usually go for one of the obscure colours at the far end of the spectrum, but in fact it was orange. The word existed. It was the name of a fruit. (Still is.) But the colour didn't take its name from the fruit until the 1600s.

Newton did many wonderful things, and contributed vastly to science. But his rainbow colour scheme was a bit of a fraud.

Image from Wikipedia: D-Kuru/Wikimedia Commons