In the last programme, Du Sautoy discussed light and electricity. I was expecting to hear how we learnt to do light by numbers. Colour is a subjective phenomenon as is brightness. Indeed, he spent a lot of time discussing the attempt to find a standard measure of brightness. But surely the key number for light is wavelength. We were told how Newton in his celebrated prism experiment showed that white light was composed of the entire spectrum. But this is still purely qualitative. Newton could put no numbers on light at all.
But today, wavelengths are bandied about at will. Lasers produce light at particle, narrowly defined wavelengths. But how do we know that?
Perhaps du Sautoy did explain how we measured wavelength and I nodded off – the programmes no longer on YouTube so I can’t check but it’s a fundamental point. Newton believed light to be a corpuscular phenomenon, as indeed quantum theory says it is, some of the time. But making light numerate depended on recognizing that it is also a wave phenomenon. The man who established that, Thomas Young, was also, in 1801, the first to measure the wavelength.
This required forgetting about subjective colours and brightness. If light is a wave, waves can interfere with each other and when they do they create patterns characteristic of the wavelength. Young arranged for a light beam to be split by two narrow slits. The two waves that emanate from the slits then interfere, with peaks and troughs where they either reinforce or cancel out. Catching the pattern on a screen, a long distance from the slits shows a banded pattern of these peaks and troughs. Simple trigonometry allows the wavelength of any particular coloured light to be determined
Democritus set this ball rolling nearly 2500 years ago when he wrote: “ostensibly there is colour ... In reality only atoms and the void”. He was only partly right. “Ostensibly there is colour – in reality only electromagnetic waves of a particular, measurable wavelength.”