I've read this answer regarding the difference between mere reflection and absorption followed by emission and I am struggling to understand some concepts. To begin with, we have some matter-light interactions that are notable:

  • Emission: occurs when an excited electron emits a photon at a random direction (that explains fluorescence)
  • Reflection: occurs when an photon elastically collides with an atom, bouncing back at the same angle (that explains reflection)
  • Absorption: occurs when an photon gets absorbed by an atom and heats it up.

The questions are:

1- Where in the atom do absorption and reflection occur, exactly? If it's in the electronic shells, then how electrons (particles) and photons (waves) possibly collide elastically?

2- Moreover, following the premise that absorption occurs on electronic shells, how does an electron absorb a photon, get energetically excited, and then doesn't emit another photon? That is, how the energy from absorption gets converted into vibrational energy and not in other photon?

Vibrational energy is also quantized. Having in mind that the quanta of vibrational energy and the quanta of light are energetically the same (both follow Planck's length), what differentiates absorption transformed into thermal energy from absorption followed by emission of another photon? In other words, if both cases of absorption follow the same criteria of initial conditions (in that case, the wavelength), how is it possible that two different determined outcomes exist?

3- How can transparent objects possibly be explained by these interactions? Why do light waves that pass through a window don't bounce off its surface (reflection)?

  • $\begingroup$ There is reflection off of window surfaces, most easily seen at night looking out a window from a well lit room. $\endgroup$
    – Jon Custer
    Nov 22 at 16:00

1 Answer 1


Light is an excitation of an electromagnetic field. So is an atom. Light is an emergent property of the free em field, while "an atom" is an emergent property of the em field that is close to the nucleus and its electrons. So there is no "where in the atom does this happen". It is only the physicist who draws an arbitrary system border: the atom here in this volume, the free em field everywhere else. Nature doesn't know about this border. It only exists in our minds and on paper. It was devised to make thinking about the problems in atomic physics easier.

In reality the electronic wave functions of an atom are falling off exponentially, so "the border" is, while not sharply defined, still a very good approximation just a few (let's say ten) atomic diameters away from a nucleus. That is the reason why we are using this mental picture.

In quantum field theory we are thinking about it the following way: "the free field" is always at "infinity" i.e. sufficiently far away from an atom (its nucleus and electrons). We don't have to concern us about the atom's influence in that region. Light in that region can be thought of as consisting of a superposition of plane waves. When these plane waves come close to the volume of an atom where their interaction can not be neglected, they get scattered. A plane wave coming in from one direction is being turned into a superposition of plane waves going out into all other directions. This is called "scattering".

In atomic physics we call measurements of scattering "spectroscopy": we illuminate an atom with a light source with a known spectrum and measure the spectrum that "comes out" after the light interacts with it. Both the light source and the spectrometer are far away from the atoms under measurement and we don't care about the details.

If, however, you wanted to treat this problem with questions in mind like "how does the electromagnetic field change during an absorption or emission process", then you are in for a hard ride. The dimensionality of the Hilbert/Fock space of the quantized electromagnetic field is infinite. There is no simple way to describe it with a finite dimensional state. You would have to make choices of a finite (small) number of base functions that describe the system reasonably well, and then try to apply your imagination to the dynamics of the superposition of these base functions. This is usually done with perturbation theory. If leading terms describe the system "well enough", then we can talk about the system in a lower dimensional approximation that is somewhat accessible.

  • $\begingroup$ Thank for your answer. It's indeed fascinating or perhaps sad that humans aren't capable of understanding such inate characteristics of nature. It's a limitation on the way our minds work. Regarding your answer, is there any feasible explanation that can explain all the mentioned phenomena in a consistent way? Every single explanation that I find on this matter ends up in inconsistencies, loopholes or is incongruent with a previous explanation. $\endgroup$
    – Marvin
    Apr 9 at 23:36
  • $\begingroup$ My physics teacher in high school used to call this limitation "working with handicap". I didn't quite understand what he meant, but it pops up everywhere in physics. Quantum field theory explains all know phenomena minus the existence of spacetime and gravitation in a consistent way, but it's usually way too hard to use in practice. We limit its use to high energy phenomena and a few low energy scenarios. It is also unnecessary. Most problems can be treated much more easily with non-relativistic quantum mechanics or classical approximations. $\endgroup$ Apr 10 at 2:03

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