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?
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 RedOrangeYellowGreenBlueIndigoViolet, the thing he called “blue” was indeed
what we would now call blue-green, 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 and magenta, 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’s 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 disc, 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 dark blue ring, and what do red and blue give?
Purple!
So as the saying goes, roses are red, violets are blue, and purple in a rainbow is a supernumerary
hue.