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What is it that make some material look black? What I mean is, why do some atoms absorb photons while others transmit them? (Correct me if I'm wrong but I think all photons are absorbed by an electron, exiting it, and shorty after the electron releases a new photon?)

But what I'm wondering, just to clarify my question, is what makes one atom absorb an incoming photon while another does not? What property is determining this behaviour?

The one that absorbs all photon will look black to our eyes (well, we can't see individual atoms, but I hope you understand what I mean), while the one that reflect it will look coloured. Or is colour not a "property" of individual atoms?

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When a photon interacts with an atom, three things can happen:

  1. elastic scattering (mirror reflection), the photon keeps its energy, and changes angle

  2. inelastic scattering, the photon gives part of its energy to the atom (heats the material), and changes angle

  3. absorption, the photon ceases to exist, the photon's energy is transferred to the absorbing electron/atom system

It is very important to understand that all three happen when light shines on an object, but different material have different ratios of those three interactions.

Now a black object is black, because it does not emit any visible wavelength photons. Contrary to popular belief, this object still can reflect or absorb photons, and re-emit them, just not in the visible range.

You are saying that black objects absorb all visible wavelength photons, and do not reflect or re-emit any visible wavelength photons. That is correct, but when you say that objects will look colored because they reflect visible wavelength photons, that is not completely correct.

In reality, non-black objects seem colored, because they both reflect and re-emit visible wavelength photons. Some of these photon are simply reflected, keeping their original energy (like a mirror), some are absorbed and re-emitted.

Contrary to popular belief, some objects can have colors, and emit visible wavelength photons, even without receiving any light. If you heat up certain materials (in a completely dark room), they might start emitting visible light, to release the excess energy in the form of visible wavelength photons.

There are other materials, that are able to re-emit visible light with a lag. Phosphorescent materials can for example do that.

Phosphorescence is a type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum mechanics. As these transitions occur very slowly in certain materials, absorbed radiation is re-emitted at a lower intensity for up to several hours after the original excitation.

https://en.wikipedia.org/wiki/Phosphorescence

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  • $\begingroup$ Thank you, I've never heard those terms before. To follow up, you said: "[...] different material have different ratios of those three interactions." What is it that determines these ratios? I think that is what is at the heart of my question. $\endgroup$ – Kristoffer Helander Jul 18 '20 at 7:58
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    $\begingroup$ @KristofferHelander It is the atomic/electron structure of the material. Certain materials, like metals, have higher ratios of elastic scattering (reflection), this is because, the atomic/electron structure is so, that the band gaps are not good for visible wavelength photon, and the atoms on the surface of the metal cannot absorb visible light. For absorption, the band gap needs to match the photon energy. If it does not match, the photon will be either elastically scattered (reflected) or inelastically scattered (heat up the material). $\endgroup$ – Árpád Szendrei Jul 18 '20 at 15:31
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    $\begingroup$ @KristofferHelander If the energy matches the band gap, higher rations of photon will be absorbed, like for a non-shiny plastic. Now the tricky part is understanding what happens in glass. Glass will do both reflection and refraction. It will have a higher ratio of elastic scattering (for visible light), that is glass can reflect a mirror image too. But glass does let the visible light through too (refract). There are two main ideas on this site what happens to visible wavelength photons when they go through glass, one says it is elastic scattering, the other says absorption and re-emission. $\endgroup$ – Árpád Szendrei Jul 18 '20 at 15:35
  • $\begingroup$ For intransparent materials it`s all about the surface. A white material, painted in black, very good absorbs light and a black body with white painting is a good reflector. A polished metal is a good reflector, a sandblasted metal a poor reflector. Furthermore the structure of the surface may play a role. Butterflys have structures that absorb some wavelengths and reflect others, and this is different for neighboring positions on their wings. $\endgroup$ – HolgerFiedler Jul 18 '20 at 15:59
  • $\begingroup$ @ÁrpádSzendrei Does colour occur per atom, or is it only molecules that have colours? What about an element such as carbon; it appears black when it is graphite, but is transparent in diamond? Isn't the band gap in carbon the same regardless of how it is structured? $\endgroup$ – Kristoffer Helander Jul 18 '20 at 16:38

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