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According to what I know, reflection, absorption, and transmission all involve the absorption of photon's energy. In the case of reflection and transmission, the absorbed energy is re-emitted in the form of electromagnetic wave (which is not in the case of light absorption). What makes the difference between them? Is it possible that three of the processes happen at the same time for a certain material?

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  • $\begingroup$ Why does transmission involve absorption? $\endgroup$ Jun 22, 2021 at 6:57
  • $\begingroup$ Transmission (presumably here) includes a change in speed (refractive index) due to absorption/re-emission. $\endgroup$ Jun 22, 2021 at 7:45

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The other replies may be somewhat confusing. It is indeed reasonable to consider absorption, reflection, and transmission all as sequences beginning with absorption of a photon. They indeed all happen simultaneously. The difference is in what happens to the energy after it is absorbed, and this is in part determined by how the photon was absorbed.

If the photon was absorbed in such a way that its energy may be quickly transferred to some other excitation (say, through electron-electron scattering to another electron state or through electron-phonon scattering to a lattice vibration), then the photon energy remains in the material, and we call it “absorbed.”

Alternatively, the photon can be absorbed into a “virtual state.” This is a state which coherently re-emits the photon. This re-emission is slightly delayed, which delays propagation through the material and results in the material having refractive index $>1$.

The photon can be re-emitted in any direction allowed by the exciting polarization; however, due to a coherent interference of waves, you’ll only have a significant probability of finding the photon in one of the classical directions of the beam (i.e. forward into the material or in the reflection direction at an interface). The photon which was absorbed/re-emitted many times and emerges out the back of the material is called “transmitted.” The photon which was absorbed/re-emitted one or more times and emerges in the reflection direction we call “reflected.”

Thus, for any single photon, it is either reflected, transmitted, or absorbed (I’m ignoring another category you might call “scattered”). For a beam of many photons, these are all happening simultaneously.

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I'm not sure why you think transmission involves absorption. Transmission is usually referred to as the absence of any interaction taking place between light and matter.

Reflection, on the other hand, is the scattering of photons, on a macroscopic

In a very simplistic sense, you can think of each photon in a beam being reflected, scattered, transmitted or absorbed with a certain probability. These probabilities are referred to as "cross-sections", and can be calculated from the parameters of the system, and are all quantum-mechanical effects.

Whether a beam is scattered or reflected is determined primarily by the wavelength of the light compared to the lattice size or number density of the material. Whether it is transmitted or absorbed depends mostly on whether it is close to a "resonant" frequency of the material (this is not the only factor involved though).

These processes can occur at the same time within a material, as ultimately, as I mentioned, there are probabilities attached. For example, a beam passing through a container of gas may be reduced to 50% its original intensity, which would mean half of the photons have been scattered/absorbed, and half have been transmitted. But of course, they cannot happen concurrently to one photon if that is what you are asking.

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  • $\begingroup$ I come across the concept that transmission of light involves absorption at here $\endgroup$
    – Saitm
    Jun 22, 2021 at 9:44

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