As far, as I understand, in quantum field theory one photon can't be absorbed by to particle systems like atoms, or can't be scattered on two single particles, like free electrons.

  1. If to consider electromagnetic radiation as distribution of changes of values of electromagnetic field at space, i.e. as a wave, how many particles then, can interact with "one same" radiation?

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*fig 1 - Example, emerald circles - em waves, pink dots - electrons

What will happen in the case, illustrated above? Will one (random?) of two electrons somehow interact (scatter) with the wave, and just after that the second one will lose ability to interact with the same wave, because the last one will disappear? Or will they both interact with em wave simultaneously?

  1. How does it corresponds with considering em radiation as a particle - photon?

My thoughts

If to consider an em wave as literally changes of electrical field strength values at some points at some time, then two electrons must be affected by same em field, and simultaneously (considering the case, illustrated above).

Some stuff is intuitively proving it, like Huygens–Fresnel principle.

Or the fact that all light sources, as I understand always emit light radially (or spherically, do not know hot to call it) in all directions, even lasers. It can be narrowed (like in case of laser), though it still will diverge.

And, considering the fact from paragraph above, I've read somewhere, and it should be logical, that if em radiation is infinitely "stretches", and at the same time its energy is constant, then, energy (and hence $E$) per area unit should decrease.

If $E$ per area is considering by someone in physics, that means that particles per can absorb not whole em wave, but only part, that corresponds to the area, that particle is occupying.

  • 5
    $\begingroup$ It doesn't seem like you know enough about the background material to formulate a valid question, I suggest consulting a textbook on QFT. $\endgroup$
    – AfterShave
    Feb 17 at 12:49
  • 1
    $\begingroup$ @AfterShave, why QFT, if question is rather about classic presentation of em radiation? $\endgroup$ Feb 17 at 12:56
  • $\begingroup$ Once a photon is absorbed it is over ... often referred to wave function collapse. For scattering, refraction, reflection there have been many discussions if its a new photon or the original photon that continues ..... but it does not make a difference o the physics. $\endgroup$ Feb 18 at 0:21
  • $\begingroup$ @PhysicsDave How many free electrons can scatter same photon simultaneously? $\endgroup$ Feb 18 at 9:26
  • $\begingroup$ The EM field is very dynamic ... in theory it is affected by all the electrons and protons etc and photons in the universe. When CO2 absorbs IR photon one electron increases in energy and the photon is gone. BUT the energy level was created by all the forces in the molecule which are from protons and electrons. $\endgroup$ Feb 18 at 16:16

2 Answers 2


In classical theory, EM wave interacts with all particles that are reached by the wave, there is no limit to the number of particles. Irrespective of the number of particles, energy obeys the law of local conservation of energy (sum of energy of EM field and energy of particles).

This is as far from EM radiation being a stream of particles as you can imagine. And even in quantum theory of radiation, EM radiation is not a stream of particles. It is a field, sometimes a nice wave, and only its interaction with matter particles manifests photon behaviour.


You might look at Thomson scattering, the classical interaction of an electromagnetic wave with a single charged particle.

Response to comment:

Trying to understand the collective interaction of a bunch of electrons with a photon is the hard way to do it. But in quantum mechanics electromagnetic radiation propagates as waves, and classical wave calculations are insightful, while quantum calculations can get lost in unnecessary abstraction. I suggest you study the variety of waves in plasmas to understand a common sort of interaction of electromagnetic waves with free electrons. Save the photon idea for the interaction with a detector.

  • $\begingroup$ But question was about multiple particles $\endgroup$ Feb 17 at 14:42
  • $\begingroup$ @Stdugnd4ikbd I thought it was about interactions with single particles. Interaction of light with collections of particles is all around you: reflection, refraction, absorption, emission... $\endgroup$
    – John Doty
    Feb 17 at 19:47
  • $\begingroup$ But reflection or refraction describes behavior after interaction of light and particles, and it involves new radiation from the particle that interacted with light $\endgroup$ Feb 18 at 9:22
  • $\begingroup$ I was asking about how many particles, electrons for example can simultaneously interact with one em wave. Absorption, as You mentioned is not a case, because free electrons can’t absorb a photon. Emission is a reverse process at all $\endgroup$ Feb 18 at 9:25
  • $\begingroup$ @Stdugnd4ikbd Isolated free electrons can't absorb photons,. but plasma can. $\endgroup$
    – John Doty
    Feb 18 at 13:14

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