“Did you know there’s no pink in the rainbow?” my brother asked in the car.
“Yeah, it always used to bug me in playschool. ‘Pink and yellow and purple and green?’ Why couldn’t they just put them in order of wavelength!’ I said. “Or teach the Richard of York gave battle in vain acronym so kids don’t just yell ‘It’s a RAINBOW!’ when they see a bunch of random colours.”
“Do you know why there’s no pink?”
“It’s because it doesn’t have a wavelength at all. It’s a non-colour, we should call it minus-green.”
“What are you talking about?” I said, and he put me onto this youtube vid, “there is no pink light,” one-minute physics from New Scientist.
Nifty, but I was still confused after watching it, so I did some digging.
The colours in the rainbow can be called monochromatic colours, or spectral colours. They have a single, dominant wavelength that trigger receptors in our eyes that send a message to the brain. We have red, blue and green receptors called cones… though it’s not quite that neat and tidy.
Light colours mix together differently then paint does. In light, all the colours together make white (and on the flip side, sunlight contains all colours), while no colours, and no light, makes black. Red and green make yellow, green and blue make cyan, and blue and red… wait for it… make magenta, or pink.
We see pink when our eyes register a mixture of red and blue light. However, there’s no wavelength corresponding to pink, because the spectrum of light (or electromagnetic radiation) is more like a long line stretching from really low energy radio waves which carry our favourite TV shows, to microwave, infrared, red …your favourite rainbow acronym… violet… ultraviolet, x-rays etc. There’s no red-pink-violet, only red-orange-yellow-green-cyan-blue-violet.
(I actually found this part of the video confusing. It sounded like we could perceive non-visible electromagnetic radiation as pink, and we can’t. We don’t see pink x-rays or pink radiowaves, and most of us are still blind to UV light. Though if you want to see the world through bee-eyes, you should check out flowers under ultraviolet light. These white flowers show a bullseye pattern, invisible to us, that directs pollinators to the centre.)
Back to the rainbow. Those spectral colours, the ones with wavelengths, are found on a Planckian locus below by following the outside curve. The pink colours are in the middle of the bottom, and are non-monochromatic colours, ones without a wavelength of their own, made by seeing two different colours at once. Hey presto, red and blue (and no green) make pink.
There’s another colour left out of the rainbow club. Where does brown come from? A dark colour, we perceive it when we see low levels of light, with a dominance towards red. It’s a dark, dirty red.