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?
In: 1
It’s not about atoms (usually) but about *molecules.* Those different woods have different molecules in them, perhaps some also the same but some different.
Perhaps to make this more clear: sure things have different molecules, but aren’t they all atoms in the end of the day? It’s just a matter of how they are arranged, right? How come this arrangement cause colors to be emitted differently?
You are correct that every carbon atom is identical to every other carbon atom (except isotopes), but what changes is how those atoms are attached toger forming molecules. You can have molecules composed of the exact same atoms, and when they are attached together differently they show different properties. One famous example of this is the molecule that gives us the flavor of spearmint, and the molecule that gives us the flavor of carroway seeds. These molecules are what we call enantiomers, meaning the only differ by the way the molecule is oriented in 3-D space.
Enantiomers exhibit a property called handedness, which we call because of its similarity to how your left hand differs from your right hand. If you have ever tried to put your left glove on your right hand, you know from experience that there is no way you can turn tour hand to make it fit the glove. This is because your hands are enantiomers of each other!
Molecular enantiomers have a special property in that they both have the same, but opposite effect on polarized light. The details of this are a bit complicated for ELI5, but the point is that small changes in molecular arrangements change the way a molecule interacts with light.
> what makes a wood chair white and a wood table red?
1. Wood stain exists
2. Wood sealant exists
3. There is more than one kind of wood because there is more than one kind of tree
> Aren’t both atom’s composition the same?
Well, the atomic scale is way too small for the effect you’re thinking of anyway; the wavelengths of light reflected vs. absorbed is more a function of the *entire* molecule. It’s not like “oxygen atoms reflect red light but nitrogen reflects blue”, it’s more like “how many double bonds in a row are there”.
But anyway, the main component in any kind of wood is cellulose, but pure cellulose is colorless anyway. The color of unstained wood mainly comes from “polyphenols”, which are these absolute rats’ nests of aromatic rings and oxygen atoms slapped together almost at random (e.g. [tannic acid](https://en.wikipedia.org/wiki/Tannic_acid)). It’s *almost* random, but it’s still performed by enzymes, which have to be coded for in DNA, and different tree species have different DNA while trees of the same species have mostly the same DNA, which is how color correlates with the kind of tree.
So, if you have a microwave, I’d like you to take a look at the front window.
If you look closely, you’ll see a mesh in the glass, a bunch of holes that lets you see inside.
What those holes are doing (or more appropriately, what the material that is creating the mesh is doing) is containing the radiation produced by the microwave’s magnetron so it doesn’t cook anything outside the microwave, while letting you see the food cook inside.
The reason that this works is that the holes are a smaller diameter than the wavelength of the microwave radiation. But not as small as the visible light radiation produced by whatever light source. This makes it act as a solid to the microwave waves, but as transparent to the visible light.
At a much smaller scale, this is what you’re seeing happen when coloration happens – the spaces between molecules, the strength of forces and how many “holes” exist for light to pass through vs how much it can let partially pass and “catch” by dissipating the energy, vs how much will just go through it entirely. A white object is going to be more tightly meshed to make all visible light bounce off of it.
It’s worth a reminder to anyone who isn’t thinking about it that all light is is a specific band of electromagnetic radiation, which includes low-energy waves like the microwave and what is used in RADAR, and high-energy waves like UV and x-ray and gamma radiation that can cause cancer by being such a small wavelength and high velocity that it can damage DNA and make it replicate wrong.
All it really comes down to is that the differences in how tight that “mesh” created by those molecules bonding is, and how much that makes it reflect vs absorb light.
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.
Latest Answers