• Member Since 28th Oct, 2012
  • offline last seen 1 hour ago


Particle Physics and Pony Fiction Experimentalist

More Blog Posts369


Rainbows and the Colour Pink · 2:38pm August 24th

Science question of today’s post: Why is there no pink band in a rainbow?

The depiction of rainbows is one of the less-frequently reported ways in which G4 MLP is superior to earlier generations.

G3 - G4

To be fair, as neither generation chooses realistic colours for the coats of ponies, there is no reason why they should for the rainbows. It could also be argued that by picking an unnatural colour spectrum, the artists are making it clear to the viewer that these are magic rainbows. And the plot requires pink to be in the rainbow. Perhaps most viewers don’t care, but it is a detail that looks wrong to a physics nerd like me.

If you can make Dashie sit still long enough to let you braid her tail, you will see the full spectrum from red to violet, but you won’t find a single pink hair. Pink is excluded from the rainbow.

This reality has led to a somewhat bizarre claim on some science blogs that pink does not exist. It’s just an optical illusion. This is undeniably true, but so are all the colours. What is it that makes pink special?

If we go into colour theory, we learn that pink is an anti-colour. Ignoring more advanced effects, our eyes have three colour sensors, each sensitive to a different range of wavelengths, letting us see the primary colours red, green, and blue. If one primary colour is missing, but the others are there, then we perceive an anti-colour. To use the names preferred by graphics people, we have yellow (anti-blue), cyan (anti-red), or magenta (anti-green).

What’s special about magenta pink, compared to the other anti-colours, is that you can’t create it with a single wavelength. A laser tuned to 580nm will show a yellow spot as it triggers both our red and green sensors, but not blue. Likewise, 490nm cyan light lies right between green and blue. But you can’t have a single wavelength of light trigger both blue and red, but not green, as the green range lies right between red and blue.

This illustrates the danger of thinking of colour as a property of light, which is how physics teachers teach it—especially those like me who like rainbows. We tell students that colour is the wavelength of light in the visible part of the electromagnetic spectrum, and go on to talk about invisible infrared and ultraviolet light. But this one-dimensional approach is not the full picture. Perhaps we should encourage students to pay more attention to their art teachers, as a better way to pick up a good intuition for how colour really works is to open up a simple drawing app and play around with the custom colour box.

Colour is not really a property of light, but a human perception—how our brains make sense of the three-channel information coming from optical nerves. What we see as white light could be a mix of red, green and blue; or a mix of three other primary colours; or a continuous spectrum. You need a spectrometer to tell.

I'll finish with a teaser for my next blog post. Colour theory turns out to an ideal analogy for particle physicists seeking to explain the strong nuclear force, which binds quarks into protons and other hadrons. While the electromagnetic force arises from a one-dimensional symmetry describing positive or negative electrical charge, the symmetry behind the strong force has three dimensions, leading to the colour charge of quarks and gluons. I’ll talk more about this theory of Quantum Chromodynamics another time.

Join our Patreon to remove these adverts!
Comments ( 17 )

Human perception gets even more fun when you have a female with corneal replacement with sapphire lens. Not only does she have dual Blue cones, giving 4 channels, but she is sensitive to near infrared and near ultraviolet as well. Try plotting the colourspace gamut for her vision. :raritystarry:

"Color is a human perception, not an intrinsic property of the universe" is definitely something that needs to be wider knowledge. It'd certainly ease the hostility certain parties have towards anti-green.

Author Interviewer

Science question of today’s post: Why is there no pink band in a rainbow?

Because they're canonically married to other ponies. :B

Pinta, have you seen the film "Color Out Of Space" ? (it as Nicolas Cage in it)
They use magenta to represent a color that doesn't exist.

5342016 Looks like you had a DOUBLE rainbow, hun.

There's a faint one right above the more noticeable one.

Horses in real life have dichromatic vision, but ponies in the show are clearly not horses. Perhaps they have 3 cone types or maybe even more. Maybe they they have no cones but instead they use chromatic aberration, the phenomenon were different wavelengths of light get bent differently by a lens, and brain processing to perceive colors.
I think it is interesting to speculate how you could have a substance that has the optical properties and fluidity of liquid rainbow. Would it be possible to make it out of normal atoms? Would it require a new force to get those properties? Would the chemistry of be such that it would be spicy to the tounge?

It's a cartoon. Just chill out and enjoy it.

That's something I learnt when I wrote my Rainbow Lasers post. Dichromatic vision does rather change your perception of a rainbow. But as our little ponies have such enormous hoof-sized eyes, I reckon their vision is different to other horses.

No, I've not seen that one yet. Magenta works well as the default error code. Like when you load a Minecraft map that was saved in an old version, and all the non-standard blocks are rendered pink.

Obligatory "mysterious colors, unlike any seen on Earth!"

🎶 If you’re wonder how he eats and breathes
And other science facts (la-la-la!)
Then repeat to yourself, “It’s just a show,
“I should really just relax, for…”

I knew "Pink is only human perception" already... But on seeing that image, I was like, "Does that explain why Pinkie finds rainbows spicy? Because they're not her color??"

Interesting! Thanks. :)

She liked the spicy food at the Tasty Treat. It must depend on the spice.

One can translate Red, Green, and Blue to Hue, Saturation, and Brightness. Pink is desaturated Red.

HSB can be very useful:

Greyscale is just Brightness. When creating TV, it was very challenging to design the electronics for just greyscale. The engineers wanted to leave room for color, when they work out the electronics. The original TV-standards (different countries had different standards based on the frequency of their alternating currents), conceptually used 5 MHz of bandwidth for the broadcasts (for technical reasons, they ended up using 5.5 MHz of bandwidth). in the late 1930s, they used the bandwidth thus:

  • 3MHz for luminosity
  • 1MHz for Sound
  • 1MHz reserved for future use

In the 1950s, the engineers added a 1MHz chromatic Channel, encoding Hue and Saturation. That is why standard-definition BW-TV and Color-TV and compatible. If the engineers would not have left a MHz for future use, Old or new but costing half the price of new Color-TV which displace only greyscale would not have been capable of displaying TV-Stations broadcasting in color in the 1950s.

Because TV devoted 1 MHz to audio, but audio required only 100 KHz, SDTVs could add stereo, other languages, and audiodescription as additional audiochannels. If the engineers would have thought to make TVs with stereospeakers and headjacks for stereoheadphones in the 1930s, TV might have totally killed MovieTheaters. As it is , after the 2nd WorldWide War, in the late 1940s, Cinema switched to widescreen, stereosound, and color for competing against TV because TV, in the late 1940s, had none of these.

Login or register to comment
Join our Patreon to remove these adverts!