The rainbow as we know it is red, orange, yellow, green, blue, violet. ROYGBV. I'm ignoring indigo because, let's be honest, indigo? But where exactly is violet? Is it at the end here? This dark blue? And what's this brighter light blue-green? Cyan, perhaps? So why don't we say the rainbow is red, orange, yellow, green, cyan, blue? ROYGCB.
Well, we actually do. And we've just forgotten. When Isaac Newton originally observed a rainbow of light split by a prism and made his labeling of the colors as red, orange, yellow, green, blue, indigo, violet, the thing he called blue was indeed what we would now call blue-green, or teal, or cyan. Reminiscent of the color of the blue sky. And what we now tend to call blue, Newton called violet, as in, roses are red violets are blue. Dark blue.
He only included indigo in his fundamental seven colors of the rainbow so that they would match the number of notes of the Western musical scale. Do re mi fa so la ti-- yeah.
Purple, magenta, and hot pink, as we know, don't occur in the rainbow from a prism because they can only be made as a combination of red and blue light. And those are on opposite sides of the rainbow, nowhere near overlapping. So there is no purple or hot pink in the rainbow from a prism. Violet is there in the roses are red, violets are blue sense, but purple is not. So then why do rainbows in the sky often look like they have purple in them?
I suspect sometimes it's an optical illusion whereby nice deep blues in small amounts surrounded by a lighter color appear purplish to our eyes. However, sometimes purple and pink really are there because a rainbow is really a rain disk. Each color of sunlight reflects back in a bright rimmed disk, all of different sizes, which together add up to make a white disk with a colorful rim.
But because light is a wave, interference from the raindrops themselves actually gives each disk multiple rings. The familiar outer ring is just the brightest. The others are called supernumerary rings and are the source of supernumerary rainbows. The smaller the raindrops, the stronger the supernumerary bows. And if the drops are the right size, the first red supernumerary ring can overlap significantly with the main violet ring. And what do red and violet give? Purple.
So as the saying goes, roses are red, violets are blue, and purple in a rainbow is a supernumerary hue.
