Romy asks, “What is color?”
Here are two puzzles about color:
- Is color cyclical (like a color wheel) or linear (like a spectrum)?
- How come some people can’t tell red and green apart?
Here’s the key: Color isn’t an actual property of any object on its own. An apple isn’t red. The apple, the sunlight, your eyes, and your visual cortex work together to create the sensation of red.
How?
Remember, light is a wave that goes around jiggling electric and magnetic things. Light comes in different jiggling speeds, a.k.a. frequencies. There are only some frequencies we can see, and those are the ones we call light. The other ones go by names like x-rays, radio waves, infrared, and so on.
Why do we see the light frequencies and not the others? We evolved to detect that specific range because it’s the most useful. It’s the range of frequencies the sun sends out. It’s also the range of frequencies that lightbulbs send out, but that’s because we built them that way.
It’s definitely useful to be able to know where there’s sunlight and where there isn’t. But vision is even cleverer than that. There’s more information we can get from these electromagnetic waves bouncing around everywhere. Incredibly, we use them to learn about the microscopic structure of materials around us.
See, when light hits any material, it jiggles the electrons. But the electrons are already jiggling — that’s just what they do. And if the frequency of the incoming light matches the frequency of the normal jiggling, they lock in. The material absorbs the light in the form of heat. This is called “resonance” and it’s similar to why glass shatters if you shriek the right note.
Fire resonates with one flavor of light, and water with another. Ripe and unripe bananas have slightly different atomic structures, so they reflect and absorb light differently. The dye in black shirts absorbs all frequencies of light — that’s why they get so hot in the sun.
If you knew which types of light lock into a material and which bounce off or pass through, you could tell how its electrons were jiggling. You could tell if a banana was ripe or not! But to do that, you’d need a complex, intricate piece of machinery to tell all the different flavors of light apart.
You’d need an eye.
The eye is a truly remarkable feat of evolutionary design. Your cornea directs incoming light through your pupil and towards the retina, in the back of your eye. Your retina is made mostly of neurons which connect back to your brain. These neurons report signals they get from “photoreceptor cells” which detect light. Photoreceptors come in two breeds: rods and cones. They’re the main characters of this story.
Rods are very sensitive. Even a tiny amount of light will trigger your rods. Rods are how you see at night, but not how you see color. That’s why you can’t tell what color things are if it’s too dark.
Cones are where it gets interesting. You have three types of cones, which each respond differently to different frequencies of light. They’re called S cones, M cones, and L cones, for short, medium, and long. Each one is sensitive to different frequencies in different ways. L cones send the strongest signal when light with a long wavelength (low frequency) hits it. If short-wavelength light (high frequency) hit it, it would send a weaker signal, and the S cone would send a strong signal.
Here’s a graph of how strong a signal each cone sends for the different flavors of light:
When any type of light hits your retina, you get signals from all three types of cones. The total strength of all the signals tells you how bright the color is. That’s how you tell black from white from shades of gray. It’s how you tell sky blue from navy blue, orange from brown.
But you figure out the rest — the “chromaticity” — by comparing the three signals. If 70% of the signal is coming from S cones, 20% from M cones, 10% from L cones, we’ll call that “blue.” If 80% of the signal is from L cones and 20% from M cones, let’s go with “red.”
We can fill in a whole chart of how we categorize light based on the signal breakdown. (We only need two axes, because the third one is just whatever percentage is left.) Something cool about this chart: if you stare at, say, the 20% L, 60% M point, then your brain will actually be receiving a signal that’s 20% L and 60% M.
The thick line curving around the outside of this shape is the spectrum of light, going from high frequency (low wavelength) in the bottom-left up and around to low frequency (high wavelength) on the right. Everything in the middle is stuff you can only get by mixing “pure” colors. For example, you get white when the three cones are sending about equal signals, which is only possible if different flavors of light are hitting your eyes at the same time.
This is the answer to our first puzzle. The spectrum shows you all the “pure” colors you can get from only one type of light, which is why it’s linear. By combining different frequencies of light, you fill in the whole color wheel. The colors that link red back to purple aren’t “pure” — you’ll never see them in a rainbow.
And the second puzzle? Some people are born with only two types of cones instead of three. Usually they’re missing the L or M cones, which means they get about the same signals for red and green. Dolphins, those poor souls, only have one type of cone. Birds have four — and, crazy enough, about 5% of women have four cones too! You’re just as colorblind to these “tetrachromat” women as dogs are to you. And it doesn’t stop there. Some animals have even more cones. Mantis shrimps have twelve! Just imagine what their world looks like.
One last thing you might be wondering: What’s going on outside that horseshoe of color? What would it look like if your brain got a signal that was 100% L? Or any other breakdown that no combination of lights can make?
I’m wondering the same thing. It’s called an “impossible color” and we have no way of ever knowing what it would look like. Maybe you should ask a mantis shrimp.
More from Romy Asks:
Originally published at milobeckman.com on April 16, 2015.
