How does matter (on an atomic scale) get its color?
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Visible light comes in different wavelengths, which refers to the lengths between two consecutive peaks on that squiggly graph you’ve probably seen before. Matter will absorb some wavelengths and not others. What you think of as “color” is what your brain perceives when the non-absorbed wavelengths bounce off of that object and hit your retina, with different wavelengths corresponding to different colors. Whether or not a wavelength is absorbed is dependent on the vibrational frequency of the atoms comprising the object.
Not so much on the whole atom as much as the electrons themselves (we’re still talking about the atomic scale here lol). Depending on how electrons in the atoms in some specific matter are arranged and move around, they will absorb photons (light) in a specific way, and maybe even reflect some back. The combination of absorbed and reflected photons are perceived by us as different colors.
If all of the photons are absorbed, it would show black. If all of the photons are reflected (depending on the how), it would show white or a reflective surface such as a mirror. Anything in between is every other color in this universe.
Light (the photons) have various frequencies and wavelengths, and our eyes detect these as colors. Photons don’t have mass, so their energy is in their frequency, basically.
Photons can interact with atoms in two ways:
* particle-like (they hit the atom and get absorbed, typically into the electrons, which move to different orbitals as a result of the extra energy they just got),
* or wave-like, where the interaction can be with the “arrangement” of the atoms, sort of like an ocean wave hitting some sort of obstacle and [bouncing/reflecting](https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQlamBxOJRqTxQU4NHKE1hH6mm8ib8sZnn7dg&usqp=CAU) off it, or [passing through](https://i.ebayimg.com/images/g/Z20AAOSwukVbFrWw/s-l300.jpg), depending on how the wavelength (“dimensions”) of the light compare to the spacing / configuration of the atoms in the material.
So basically transparency, reflections (mirror effects), as well as color effects where some wavelengths are absorbed or blocked, and some pass through or get reflected to your eyes where you can “see” them, are a result of how light interacts with materials.
The matter in itself doesn’t have colour, it’s about the way it interacts with light. There’s no such thing as a green atom or a blue atom, it’s about how different lengths of light waves bounce off it and how we see that. That’s why different animals see different colours
On an atomic scale, atoms have different energy levels that electrons “sit in”, when energy from a photon hits an electron, based on the energy of the photon it will cause the electron to jump to a higher energy level (or several higher)
When the electron then releases this energy, it jumps from the higher energy level to a lower energy level, and depending on which levels it jumps between, will release a certain amount of energy as a photon which corresponds to a certain wavelength of light
So when you hear about objects really absorbing all but one color of light, what that means is that if something is purple, it’s absorbing all the red and yellow and green (etc) light to jump to the higher energy levels, and then when it jumps back down it’s expelling the energy of a “purple” photon
Most things will release a few specific wavelengths of light (ie have multiple energy levels it jumps down between), that our brains meld together into a final “color”, but you can actually see the individual colors using a fairly cheap [spectrometer](https://www.teachersource.com/product/hand-held-spectroscope?gclid=EAIaIQobChMIxInfnf6d9wIV-AiICR06qwN7EAkYASABEgKZnPD_BwE)
There’s reasons why different elements prefer to jump to/from certain energy levels but this part would be beyond my knowledge
White light is photons of all kinds of energy levels.
Electrons orbitting a nucleus (outdated model, but sufficient for this case) can have only certain energy levels, for instance 5 and 9, but not 6, 7 or 8.
An electron can only absorb a photon when the energy of the photon is the exact amount it needs to get to a higher energy level.
Now suppose we shine white light (for simplicity: photons with energy 2 (red) and 4 (blue) ) on our atom. It will absorb only the 4 photons, so it will apear red.
For low concentration atoms and molecules (such as gas, ions in solution or impurity ions in a solid), the electrons can only move between a few filled up orbitals and the empty orbitals. Each move corresponds to a precise energy and wavelength (simply put, a portion of the light that corresponds to a color). The combination of absorbed wavelengths form the color.
For high concentration atoms and molecules (such as the main components of a solid or liquid), the electrons can move between filled up, semi-filled and empty bands. Any move corresponds to a wavelength band (simply put, a portion of the light that corresponds to a band of color in the spectrum, violet to red passing by blue, cyan, green, yellow, orange). The moves inside the semi-filled band correspond to reflection if there is a semi-filled band (only metals have semi-filled bands). The reflected and absorbed wavelengths form the color.
When light is absorbed and an electron moves from one orbital to another, or from a band to another, the electron will move down afterwards by transferring the energy to heat, or sometimes by emitting light.
When light is reflected by having an electron move inside a semi-filled band, there’s some chance the electron loses its energy and doesn’t reflect the light. Then, the light is absorbed. The more resistance to electricity the material has the more likely it is that it absorbs light. That’s why high conductivity metals like silver have the highest reflectance.
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