Newton's unsound theory of light

Everybody knows that Richard Of York Gave Battle In Vain. A few of us may even recall how ROY Got Blotto In Vienna.

These charming mnemonics have reminded generations of schoolchildren that red, orange, yellow, green, blue, indigo and violet are the seven colours of the visible spectrum, as demonstrated by Sir Isaac Newton with the help of two glass prisms.

But are there really seven distinct colours, or was the father of the scientific method guilty of some unscientific reasoning?

It was well known long before Newton's time that clear glass could transform white light into many colours, but it was always assumed that these were somehow produced by the glass itself. Then, in 1666, Newton brought his colossal mind to bear on the subject, and began studying the spectrum in detail.

He decided the colours were actually the constituent parts of white light, which spread apart because they followed slightly different routes through the glass. When he placed a second prism in their exit path, he was unable to spread them further and from this he concluded that the colours were "pure".

But how many colours were there? In his early lectures, Newton spoke of the spectrum as "an indefinite variety of intermediate gradations of colour". But by 1704, when his Opticks was published, he had fixed the number at seven, in the face of critics who claimed to have divided them further.

While the number of gradations is indeed infinite (the human eye can discern about 10 million shades), the visible spectrum can really be divided into five main colours. However, Newton, like many of his contemporaries, was a believer in the Pythagorean doctrine of natural harmony, central to which was the idea of a musical basis for the motion of the planets. Because there are seven intervals in the musical octave, Newton was determined that there should be seven colours of light, and proceeded to bulk out his "pure" colours with orange and indigo, both of which are merely the halfway points between their nearest neighbours.

Thus was poor Richard condemned to an eternity of futile conflict.

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