I understand the underlying basic concept of colours from my Biology/Chemistry classes. “Things” absorb some wavelengths, or emit other wavelengths (something like that).
But like, why and how? For instance, what makes a wood chair white and a wood table red? Aren’t both atom’s composition the same? How can they emit different colors at all?
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So start with asking “what is light?” Visible light is a narrow spectrum of what we call electromagnetic radiation, which comes in different wavelengths. Depending on the frequency, EM radiation behaves really differently when it interacts with matter. Some passes through most of it (like radio waves), some only interacts with certain types of it (like X-rays), some causes electrons to get excited (ionizing radiation), etc.
What we call visible light tends to get selectively absorbed by it, with the rest being reflected. That’s just what that frequency of EM radiation does when it comes into contact with molecules and atoms. Different molecules interact with those frequencies of EM radiation differently, and so reflect different frequencies of it. It’s sort of like how black shirts absorb more infrared (heat) than white shirts; it’s just how it works out.
What we call “sight” is an evolved mechanism for detecting EM radiation in a very narrow spectrum — “visible light” — with our eyes. A long, long, long, long time ago, evolution led to creatures that could detect EM radiation with specialized cells, and it turned out that being able to see some frequencies of EM radiation is more useful than others. The ones that humans can see tends to be a spectrum of EM radiation that doesn’t get absorbed by water very well, which is helpful not only because our eyes happen to have water in them (so we can use water as a lens), but also because that is in general a pretty useful swath of the spectrum for detecting physical size and location and things like that. Note that many animals have evolved different kinds of color vision than us, having brain hardware for interpreting either more (birds can see into the ultraviolent) or less (dogs see fewer colors) of the EM spectrum.
So that’s what color “is”: a sensation produced by the mind to interpret EM radiation within a certain narrow range of the spectrum, which happens to be pretty useful for surviving on Earth. You could easily imagine an alien who could see in other wavelengths, and indeed there are, again, animals that can see or “see” (in that they don’t use eyes) in other wavelengths (pit vipers can “see” in infrared, for example, through special organs in their face — we can only guess what it would be like to be able to perceive things that way).
The interesting thing, here, is that we have almost no way of describing the _sensation_ of color in anything but relative terms. So you couldn’t really describe what “red” is like to someone who had been blind their entire life; it’s sort of a fundamental experience, and we don’t really have a language for those. We can tell you what wavelength it is, but that doesn’t actually describe the experience.
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