This year’s “Flame Challenge” asks scientists to explain color in terms an 11-year-old can understand. The rules limit answers to either 300 words of text or a 6-minute video. 300 words is ridiculously short, so video is clearly the way to go. Of course, I’m not much of a video expert, but then, one of the finalists last year (when the question was “What Is Time?“) was just a guy talking into a webcam, and hell, I can do better than that. So I did this:
(This is, obviously, why I was fooling around with looking at the spectrum of light from my laptop a little while back…)
The approximate text of the video’s narration, if you don’t want to watch it, is:
The question “What is color?” is actually pretty sneaky, because what we think of as color is a combination of two sciences: the physics of light, and the biology of sight. They interact in weird ways, making questions about color tricky to answer. There’s no better example of this than the color purple. Every kid in kindergarten knows that if you mix red and blue you get purple, but physics tells us that there’s no such thing.
In physics, what we describe as color has to do with the wave nature of light. If you shake a charged object or a magnet up and down, it creates a disturbance that travels through space and causes other objects to move up and down as well. We call this a wave in the electromagnetic field (which just means “a wiggling disturbance that affects electric charges and magnets”). That wave is what we know as light.
Physicists describe waves in terms of how fast they wiggle up and down—the frequency—or the distance it travels during one trip up and down, the wavelength. To a physicist, visible light is just a particular range of wavelengths—between about 400 and 700 nanometers, or a few one-thousandths of the width of a hair—and the color of the light is related to the wavelength. Different colors of light correspond to different wavelengths.
We measure light using a tool called a spectrometer that tells us how much light at different wavelengths something produces. A red laser produces a lot of light at 660 nm, and very little at other wavelengths. Green light has a kind of medium wavelength—around 540 nm—as we see with this green laser, and violet light has the shortest wavelength of anything we can see, around 400 nm. In physics, color means wavelength, and every color is its own unique wavelength. Violet light, like this 405-nm laser, is not a combination of red and blue, but its own thing.
So where do we get the idea that purple is red plus blue? Well, if we look at the spectrum of a picture of that violet laser, on my computer, we don’t see a sharp peak at a single wavelength, but a broad mush, with light at lots of different wavelengths.
So, why is that? That’s where biology comes in. We don’t have spectrometers inside our eyes to measure wavelengths; instead, we see colors of light thanks to special cells in the back of our eyes that send a signal to our brain when they detect light. There are three types of these, each sensitive to a particular range of wavelengths. There are “long wavelength” cells that are most sensitive to orange-red light, “medium wavelength” cells that are most sensitive to yellow-green light, and “short wavelength” cells that are most sensitive in the blue. When one of these cells gets hit by light in the right range, it sends a signal to the brain saying “Hey, I see some light!” The brain collects signals from all three types, and puts them together to decide what color light you’re seeing.
The ranges of these cells overlap, so yellow light will make both the long- and medium-wavelength cells respond, and green light will make both short- and medium-wavelength cells respond. If the long-wavelength cells respond more than the medium-wavelength ones, then it knows the light you’re seeing is more red than yellow. If the short-wavelength cells respond more than the medium-wavelength ones, then it knows the light you’re seeing is more blue than green.
This is why the color-adding trick works. You can fool the eye into thinking it’s seeing a particular wavelength of light by combining other wavelengths in just the right way. Green light makes both the short-wavelength and medium-wavelength cells in your eye respond, but so does a combination of blue light (which makes the short-wavelength cells respond) and yellow light (which makes the medium-wavelength cells respond), which is why we say that blue and yellow add to make green. With the right mix of colors, you can fool the brain into thinking that it’s seeing any wavelength of light that you like.
The displays used in TV’s, computer monitors, and smart phones use three sources of light: one mostly red, one mostly green, and one mostly blue. When my computer tries to reproduce the color of the 405nm laser, it uses a mix of these; mostly blue light, with some red and even green mixed it.
This color mixing also allows us to “see” colors that don’t exist in physics, like purple. There isn’t a single wavelength of light that corresponds to what we see as purple—rather, it’s a mix of mostly-red and mostly-blue light. We can even see this by mixing together the red and blue lasers we looked at before: in the region where the two beams overlap, our eyes see an entirely new color, one that physics tells us doesn’t really exist.
So, what is color? It’s what happens when the physics of light combines with the biology of sight to make the world that we see, in all its glory. Even if some bits of that exist only in our minds.
There are pictures galore in the video, but I’m not uploading them all here. Who do you think I am, Ethan Siegel? The “featured image” up top, showing overlapping red and violet lasers, is as much as you’re going to get…