This is a great thread, I'm learning a lots!
On the subject of rainbow colours and wavelengths, my aged retired tool maker neighbour once told me that when using a square or straight edge to check a work piece, he would hold it up to a light, the colour of the light (blue to red) through any gaps would tell him how much he had to take off, he reckoned it was pretty accurate and quicker than any other way.
I spent many years in construction, when the UK supposedly went metric (there are plenty who still haven't!) the schools taught youngsters to express measurements as metres, decimetres, centimetres and millimetres.
We builders only use millimetres, so this drove us nuts as when a youngster was at the other end of a long tape measure on a noisy job and you just need clear measurements, like '7-4-8-3' shouted at you against the wind. Easy to understand, just like '24 foot 6 and a half inches' used to be!
Bernard