The retina has cones with three opsin pigments that are sensitive to different colour bands, peaking in orangish-red, green and deep blue. Perhaps another culture would have come up with a different number of colours and a different mnemonic. In the UK, we are taught the mnemonic “Richard of York gave battle in vain”, so we look for the corresponding colours in the rainbow. While this was simply one of Newton’s mystical beliefs – others included alchemy and the philosopher’s stone, which could turn base metals into gold – it has become part of our cultural inheritance.
We have seven days of the week, seven natural notes in most Western music and, in Newton’s time, only seven planets had been discovered. It is said that Isaac Newton only perceived five colours in the rainbow and added two more (orange and indigo) because the number seven had mystical significance. Mike Follows Sutton Coldfield, West Midlands, UK I have never been able to see indigo and violet as separate colours. I am a volunteer at Woolsthorpe Manor, Newton’s family home, and have often demonstrated his prism experiment to visitors. He added orange and split purple into indigo and violet. This sort of mysticism fascinated Newton as much as science, so he thought there must be seven colours in the rainbow. However, the number seven had long been considered mystical, denoting perfection and completeness. This book was published in 1664, just before Newton started his experiments. For example, in his book Experiments and Considerations Touching Colours, Robert Boyle described the spectrum he produced with a prism as “denoting the five consecutions of colours Red, Yellow, Green, Blew, and Purple”. Prior to that, the spectrum had been thought to have five colours. In reality, there aren’t seven distinct bands, but multiple colours blending and shading into one another.Īround 1665, Isaac Newton performed experiments with a prism producing a spectrum in which he identified seven colours. Our perception of a rainbow is coloured (pun intended) by our expectation that there are seven colours.
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