When Isaac Newton allowed the sunlight passing through a hole in his blind to hit a glass prism that split it into different colours, he wasn’t the first to see such colours. In fact, he’d probably seen them as a child, reflected from the bubbles in his bath, and of course in the colours of the rainbow, which appear when white light is split up by being bounced around inside raindrops.

Newton was the first, however, to think of using a second prism to put the colours back together again, and hence to prove that white light is made up of many different colours. But how many?

It depends on your imagination. To the mediaeval mind, there were just five colours in the rainbow: red, yellow, green, blue and violet. But Newton added two more – orange and indigo – because he believed that the harmony of colours in the rainbow must be similar to the harmony of notes in a major musical scale. Seven steps in the scale; seven colours in the rainbow. Newton looked for them, and he found them. It rather puts the lie to the old saying that artists see what they believe, but scientists believe what they see.

Image Attribution: Christine Matthews

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