The Color Purple

Ok, I’ll admit this post was inspired by a political article about a state turning purple. But,  it’s not about politics.

It’s about wavelengths and human perception.

What is purple anyway? Historically it was the color of royalty, in part because the secret to making a dye of that color was a closely guarded secret, and expensive to boot.

Technically, it’s a combination of red and blue, but when seen as a spectral color (e.g. splitting white light with a prism) it has a wavelength between 380-420nm and is called violet.  So we can see violet as a distinct wavelength or we can see purple as the combination of two colors. In fact, we can’t really create violet on a computer screen. Any purple you see in this post is a combination of red and blue (if we’re talking RGB space, 127,0,255 btw).

But what are we really seeing? That’s the part that fascinates me.

It’s tempting to think our senses accurately perceive the world, but the truth is that they at best our brains form an approximation of the world, and color is one of them. For example, in our eyes, most of us only have 3 types of cells, cones, that are sensitive to light and they are sensitive over a different range of wavelengths; Graph showing the wavelengths the S, M, L cones the human eye are sensitive to. Looking at this graph (courtesy of Wikipedia Commons) you’ll see that pretty much only the blue cones are reacting to wavelength of violet.  But, if you mix red and blue, the cones react a bit differently, and we see purple.

So, we’re seeing a color that arguably isn’t what we might think: i.e. violet is not purple, even if we normally equate them and for all intents and purposes they can look the same to us.

Even then, we’re missing out. For example, in most people their lens blocks UV light (which is a good thing in the long run). But some people who have had their lens replaced w/o UV blocking materials can see into the UV. And of course some people, mainly men (because two cone genes are on the X chromosome) are colorblind. But even more rare (at this time there appears to be a single individual who has been identified) is tetrachromacy, there there’s a 4th cone, that is most sensitive between the green and red cones above.

Even more weird are “impossible colors” such as Bluish-Yellow, or Reddish-Green. I’ll let you Google those, but they’re pretty cool.

So, next time you see the color purple, stop and think about what your brain is really seeing, or not seeing. Are you actually seeing violet or purple?

P.S: we’ve got nothing on the Mantis Shrimp, which has between 12-16 different types of cones! That said, it doesn’t seem to be much better at picking out colors than your or I. But I do have to wonder what it would be like to have a higher cone count.

P.P.S: A story I once read, that I’ve never been able to verify is that during WWII the English experimented with lighting some airfields with lights in the near UV range (i.e. just outside of normal human vision) because they had discovered some folks could see into the near UV range. The idea was that by using such fields at night, without any normal light, they could safely operate at night without the Germans bombers seeing them. Apparently the idea fell apart when further research discovered that the people most likely to be able to see into near UV were blond-haired, blue-eyed of German descent.  I’d normally write this off as conflating a number of myths, including the carrots make your eyes better, but I’ve seen elsewhere that apparently blue-eyed people ARE more likely to see into the UV range (from what I’ve read, it appears some UV may leak in through the iris). I’d love to find more details on this particular idea (which until I do, I will consider a UL). That said, I’ve got to say, I’ve found my night-vision is far better than most people I know. I guess that makes up for my normal vision for which I need glasses!

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