Understanding Rainbow, What Are The Real Colors?

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.

Not all coloured patches in the sky are rainbows. Rainbows are formed by refraction and reflection of sunlight by raindrops. When sunlight passes through ice crystals, halos form and they are mostly coloured. Small cloud or fog droplets diffract light to form ringed glories, coronae and the jumbled colours of iridescent clouds. (Atmospheric OpticS, UK)

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, 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’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 violet 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.

Source: minutephysics.

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