Mixing colors: pigment vs. light
Today we will address another topic in a list of “things I’m kind of ashamed I don’t understand considering I am a professional scientist of sorts” (please make suggestions!).
Why is it that when you mix light blue (cyan) and yellow paint you get green paint, but when you mix cyan and yellow light you get white light?
Unlike with yesterday’s analemma post, where I couldn’t find a satisfactory write-up on another blog, today’s blog is actually pretty nicely explained and beautifully illustrated here. I will crib their illustrations and summarize the explanations but it’s really out-and-out plagiarism for the moment.
First, you’ve got the so-called “hue wheel” (which sounds more sophisticated than “color wheel”, don’t you agree?):
This is illustrating the following. There are three basic pigments: yellow, cyan and magenta. There are three basic colors of light, namely green, blue, and red. And if you mix the fundamental pigments pair-wise (as in, you get paints and mix them) you get the fundamental colors of lights.
And vice versa as well, although this time you’re mixing as in splicing them together but keeping them separate, like we use pixels on our screen. This means, specifically, that you can combine green and red to get yellow. That’s majorly unbelievable until you see this miraculous picture, also from this webpage:
See how that works? I just can’t get over this picture. The little piece of yellow on the left is just stripes of green and red. Really incredible. The purple I get because it’s blue and red just like it’s supposed to be.
The first thing to understand is that this isn’t just a relationship between us and the object we are looking at. It is instead a three-part relationship between us (or more specifically, our eyes), the object, and the sun (or some other source of light, but it’s more traditional in explanations like this to use fundamental, macho objects of nature like the sun).
Nothing can happen without a source of light. Which begs the question, what is light anyway? Again a picture stolen from here:
The prism separates the white light into various wavelengths, where red is at 700 nanometers and violet at 400 nanometers. More on the visible spectrum here. Note that the hidden difficulty here is why a prism does this, which is explained here.
So when an apple looks red to us, we have to imagine white light from the sun hitting that apple, and the key is that the skin of the apple is absorbing everything except the red light:
That thing on top is the sun, and the thing on bottom is your eyeball. The point is the red part of the light is reflected off the apple skin into your eye. And even though white light from the sun is the whole spectrum, we are denoting it when just the fundamental three colors of light because other colors can be made from those. And this can be corroborated by looking at your computer screen with a magnifying glass, where you will see that the white background is actually made up of little pixels of green, red, and blue.
By the way, we are again sidestepping the actual hard part here, namely why some surfaces such as apple skins reflect some colors like red. I have no idea. But I don’t feel as guilty about not understanding that.
Finally, back to the first question, of why cyan and yellow paint make green whereas cyan and yellow light make white. Turns out the light one is actually easier, since our second picture above shows us that yellow light is actually a mix of red and green, and when you add cyan, you now have all three fundamental colors of light, which gives us white light.
If you have cyan paint, then it is reflecting blue and green light, so absorbing red light. If you have yellow paint then that’s a material which is reflecting both green and red, so absorbing blue. For some weird reason (a third moment of stuffing things under the rug), the mixture of the paint is additive on absorbing things, so absorbs both blue and red, leaving only green reflected.
In the end we get a kind of mini De Morgan’s Law for color.
I’ve convinced myself that, modulo the following three questions I understand this explanation:
- How does a prism separate white light into the colors really?
- How do different surfaces decide which lights to reflect and which to absorb? And a related question from Aaron, why do colors fade when they’ve been in the sun?
- Why is “absorbing light” an additive procedure when you mix materials? I feel like if I understood 2 then I’d get 3 for free.