I understand that light is simply electromagnetic waves that our eyes interpret as light.
And that RGB is a combination of red, green, and blue light.
But how does combining three separate lights actually result in a different color? Superimposing waves shouldn’t change the frequency/wavelength of the light at all, right? Is it just our eyes doing some black magic?
In: Physics
The short answer is it doesn’t. True yellow light has a wavelength of ~550nm. The “yellow” that your computer screen sends at you is just a combination of red (~700nm) and green (~520nm).
You’re correct that our eyes are doing black magic. We have three types of ~~light-sensing~~ (EDIT: color-sensing *) cells (called “cones”): L, M, and S, named based on what wavelength they’re sensitive to. (L is mostly sensitive to red, M mostly to green, S mostly to blue).
Colors like (true) yellow stimulate both the L and M cones, but our nervous systems can’t tell the difference between yellow and a mixture of red+green. That’s the fact that RGB displays leverage to their advantage.
Bonus fun fact: Magenta (red + blue) doesn’t correspond to any (pure) color, it’s just an invention of our brains to represent the L+S cones being stimulated at the same time.
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