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When photons hit matter, the electrons in that matter get excited (if the energy of the photon is sufficient to excite the electron to a higher energy state). But we know that the electrons are unstable at higher energy states and hence they jump back to the ground state either in one step or multiple steps thus leading to re-emission of photon(s) of same or different wavelengths. Thus, reflection of light occurred. But if this is true how can absorption of light ever be possible? An electron will always re-emit a photon and hence only reflection of light would take place. So, my question is how is absorption of light even possible? How will the electrons convert the energy of the photon into thermal energy if they always re-emit the photon to get back to their ground state. Shouldn't absorption of light be impossible then? Please answer. I am so confused.

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"An electron will always re-emit a photon "

Here's where you go wrong. An excited atom or molecule can decay in ways other than radiative. Some energy can be transferred to vibrations, for example. A photon of energy lower than the excitation energy is emitted, the remainder going into a vibration mode. The same can occur to the emitted photon, causing emission of an even lower energy photon. This continues until all of the initial energy is in vibration modes, or a remaining photon has an wavelength outside of the visible spectrum.

Note that electrons do not emit photons. The atom or molecule does. Keeping this distinction in mind will help understanding in the long run.

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  • $\begingroup$ Isn't phononic coupling just an effective interaction mediated by photons? (So would it be accurate to say that the system momentarily 'emits' a photon or a cascade thereof in exciting quantum or semi-classical vibrational modes?) Also, could it be said that an 'electron' emits a photon when speaking in the non-interacting electron approximation? $\endgroup$
    – TLDR
    May 2, 2021 at 3:59
  • $\begingroup$ Really nice answer. $\endgroup$ May 3, 2021 at 16:13
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You are correct that net absorption of light will not occur in objects that are in complete thermodynamic equilibrium. For such objects, the Principle of Detailed Balance demands that all microscopic processes are in balance with their corresponding inverse processes. Thus, just as much radiative emission as absorption.

But net absorption can certainly take place if an object is not in equilibrium. For example, if you place an object in direct sunlight, it will absorb more radiation than it emits until it attains a new, higher equilibrium temperature.

It should also be realised that emission processes do not have to occur in particular directions. Thus even for objects in equilibrium, we can get the appearance of absorption simply because light is absorbed out of a beam but emitted in all directions.

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Not an expert, but let me try.

When you have a single atom, then the only possible change in internal energy is essentially electronic.

When you have a molecule, you get internal mechanical states as well, like vibrations, which are spaced much closer together in terms of energy.

In bulk matter, like a crystal, you have numerous mechanical states, and you also have collective electronic states that are so close together in energy that they can be treated as continuous.

Excitation of many of these typically result in non-radiative relaxation.

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The subjective answer:

Whenever you or any other system needs a nudge, an atom coupled to the vacuum is always available to act as a source or scatterer of photons, for however much time we are given to experience. If you observe a photon, or witness the trace of one, that photon has for all intents and purposes been irreversibly absorbed, transferring its momentum and energy to the observer/apparatus.

The long answer:

After a photon is absorbed, it can often be re-emitted in essentially any direction. This is what (probably) gives rise to the subjective experience of time passing, and also happens to be the underlying principle of laser cooling. In more general contexts, you can imagine molecules or materials absorbing light, causing a transition to a higher orbital, which then influences subsequent semi-classical dynamics of the ionic coordinates, which may involve distributing the energy across a more diffuse spectrum involving or arising from the coordinates of a many-particle system, and possibly emitting a photon or series of photons later at a completely different wave number(s) and/or frequency(ies).

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