I was thinking about paint and dye and how they are different colors but made of the same stuff(?). Like I know that different material absorbs different light and the result of that is a certain color reflected back to our eyes, e.g. if a material absorbs all light it is black and that light energy become heat. But what is it that causes different things to absorb light differently? Why can a paint be yellow and then the same formula be red? I don’t get it. My brain is so fried. What is doing the reflecting and absorbing? Sorry if this counts as a loaded question, or if it’s just a that’s-just-the-way-it-is type of thing. Honestly I’m not even sure want I expect the answer to be.
In: Physics
So… since you already got the more top level explanation of light being a spectrum or composition of different colors, and the one that you SEE is the one that is reflected… I’ll go a bit further down.
Light is electromagnetic waves, conceptually. A wave of energy in the magnetic/electric field. The energy the wave contains is related to its wavelength (which, determines its color). Additionally, while it is a wave, it also is essentially a “packet”, like a particle, in how it behaves on some points.
Different materials will interact with these units/waves of energy in different ways. Some will bend/reflect them according to their optical properties, but also on a very low level there is something else going on. The basic idea is, the electrons surrounding atoms can occupy different “levels”, each representing a different state–“excited” if they are above the lowest possible energy state they can take. The set of states available to them are determined by the molecule they are a part of, basically their immediate environment. Incoming electromagnetic waves can be “absorbed” to move these electrons up in energy states, but only if the energy of the wave is at the right level. If it too high or too low, it just passes through, as it cannot be absorbed. This is why different materials absorb things differently.
Aside: Organic/carbon-heavy molecules are messy and can tend towards black (or sometimes yellowish in part due to sulfur), metals and organic molecules with highly dislocated electron bonds can be quite colorful, and many pure substances are just white. But all of them generally have characteristic wavelengths they absorb well, just often if they were “colorful” it’s not in the visible spectrum.
So in paint, you are generally looking at very specific dyes and pigments which are very strongly colored–molecules with structures that are good at absorbing chosen wavelengths. Even if these pigments/dyes/etc make up only a *tiny* amount, they are really good at absorbing specific wavelengths, so just a thin coat, tiny diffusion through a larger volume, or stain on another material (dyeing) is enough to produce a very noticeable change in light interaction.
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