How many colours are there in a rainbow?

Warm-up exercise : what are the colours associated with conservatives/right-wing politicians and liberals/left-wing ones? Does it depend on the country?

 

Question of the day

How many colours are there in a rainbow?

https://www.scienceabc.com/nature/what-how-many-colors-of-rainbow-pink.html

 

 

 

https://www.lenfisherscience.com/how-did-the-rainbow-get-its-colours-its-all-down-to-newton/

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.

* In music theory, a scale is any set of musical notes ordered by fundamental frequency or pitch. A scale ordered by increasing pitch is an ascending scale, and a scale ordered by decreasing pitch is a descending scale.(source wikipedia)

 

Caption: Newton's color circle, from his book Opticks of 1704, showing the colors he associated with musical notes. The spectral colors from red to violet are divided by the notes of the musical scale, starting at D. The circle completes a full octave, from D to D. Newton's circle places red, at one end of the spectrum, next to violet, at the other. This reflects the fact that non-spectral purple colors are observed when red and violet light are mixed. (Source: wikipedia)