In conductors the valance and conduction band overlap so there are free electrons. If a radio frequency photon hits one of those electrons (as in the case of an antenna), what will that electron do? Will it absorb the photon and oscillate up and down the antenna, or will the photon scatter or something else? There is a lot of information on how higher energy photons interact (like the photoelectric effect), but I want to know how a low energy photon interacts with conductor electrons.

An electromagnetic wave consisting of multiple photons will move the electrons up and down the antenna depending on polarization, but what will a single photon do?


2 Answers 2


Quantum mechanics is the underlying basic theory of matter for the mainstream models of physics. All other theoretical models can be proven mathematically to emerge from this quantum level . The basic tenet of quantum mechanics is that all observations and measurements depend on the probability of an interaction happening given by the wavefunction describing the system. A simple example is the hydrogen atom where the (x,y,z,t) of the electron around the proton is given by the wavefunction of the solution of the quantum mechanical equation for the hydrogen atom.. This means that to get the location of the electron around the proton one has to do many measurements and plot the distribution of the probability to find the electron. For complicated atoms and molecules one gets orbitals, not orbits. In the case of matter in bulk in addition the potentials entering by the attractive forces between molecules create lattices , and again their behavior is probabilistic.

A conductor is a system that has been modeled with quantum mechanics, and in the quantum mechanics of solids the electrons are not free, they are in various bound states. This link describes the quantum mechanical model of the band theory of solids:

band thery

Individual electrons are either in the conduction band or in the valence band. In the valence band the electrons are in energy levels of atoms and molecules. The energy levels in the conduction band are occupied by electrons that are bound to the whole lattice of the conductor, but still they are bound . This means that in order to leave the solid a photon of the correct energy and higher can kick them out of the quantum level (photoelectric).

Photons of lower energy but with the correct energy difference can change the energy level of the lattice bound electron, the low energy photon interacts with the whole lattice to do that. It is the same as with a photon raising the energy level of the electron in a simple atom.The conduction band though has so close energy levels of the lattice that the electrons can be modeled to be free within it, but the individual photon electron interaction goes through changes in the occupation of lattice energy levels.

You ask in the comments:

How does this translate to motion of the electron?

A single electron does not move. It just has a probability of being "found" in a given direction and position. It is the accumulation of electrons positions that can give the motion.

i still dont understand how change in the lattice energy level correlate to direction of the electron.

If one could solve the complicated potentials of a lattice when interacting with the photon, the wavefunction would have the probability of finding a single electron in the direction of the macroscopic current that emerges as the classical theory of electricity and magnetism.

In the simple atom example you gave, absorbed photon changes the electrons wave function so where it could be found. is something similar going on

yes, the quantum models are "averaging" keeping the quantum mechanical nature of the lattice of atoms and molecules.

  • $\begingroup$ Could you expand on your answer? How does this translate to motion of the electron? i still dont understand how change in the lattice energy level correlate to direction of the electron. In the simple atom example you gave, absorbed photon changes the electrons wave function so where it could be found. is something similar going on? $\endgroup$
    – dex
    Commented Jun 3, 2022 at 19:47
  • $\begingroup$ First of all thanks for your answer however i still have one thing i dont really get. In an atom when the electron absorbs the photons energy it changes the wavefunction so where and how likely the electron could be found. however this doesnt change over time In the atom example. The wavefunction does not oscillate with a frequency and stays in one shape until electron falls back in energy level. In the antenna electron, the probability density should be changing over time right? At one point in time it is more likely to be found up the antenna and later down. why are they different? $\endgroup$
    – dex
    Commented Jun 6, 2022 at 4:29
  • $\begingroup$ @dex Lets be clear. It is not the electron that absorbs the photon in the atom or any bound state.The difference between the steady state solutions of the atomic and molecular wavefunctions and the wavefunction instantaneously describing a solid lattice is that since the input is time dependent, ( the time the photon hits the lattice) the time dependent solutions of the quantum mechanical equations should apply. $\endgroup$
    – anna v
    Commented Jun 6, 2022 at 7:22
  • $\begingroup$ @dex I thought about it more, and photon atom interaction would also need time dependent wavefunctions, except we are so used with working with the bound states and the stable energy levels of atoms that we ignore of how the photon interacts with the atom, I think it would need quantum field theory to describe mathematcally, but it is not necessary in order to understand atomic structure. In the case of the flow of low energy photons impinging on an antenna one cannot ignore the time dependence, and the collective quantum models must have it. $\endgroup$
    – anna v
    Commented Jun 6, 2022 at 16:57

Let's talk about how a radio wave is created. Then we will be able to imagine how this kind of EM radiation affects the surface electrons on the receiver rod.

To create a radio wave, the electrons in an antenna rod are periodically moved back and forth. During each of these synchronous accelerations, polarised photons are emitted. Their electric field components are aligned parallel to the antenna rod. The magnetic field component is oriented perpendicular to it and perpendicular to the direction of emission. What we get is a rising and falling intensity (including sign change) of the two fields, caused by very very many polarised photons moving away from the source at the speed of light.

At the receiver, everything happens the other way round again. The incident (polarised!) photons cause the Lorentz force and their energy is converted into kinetic energy of the electrons. The electrons move back and forth synchronously with the radio wave. The photons are absorbed by the electrons in the process.

To put it very clearly, the frequency of the emitted photons depends on the electrical voltage of the source and the electrical resistance of the rod material. The frequency of the radio wave, on the other hand, depends on the frequency of the wave generator.

At the receiver, different radio sources cause overlapping accelerations of the surface electrons. Plus a noise of the other incoming photons. The task of the receiver electronics is to filter out the desired carrier frequency (and then decode the imprinted information).

  • $\begingroup$ Thanks for the answer. You said "frequency of the emitted photons depends on the electrical voltage of the source and the electrical resistance of the rod material." doesnt the frequency of emitted photons depend on the frequency of the wave generator?Why do they depend on source voltage and resistance? $\endgroup$
    – dex
    Commented Jun 10, 2022 at 0:22
  • $\begingroup$ Look at the movement of a single electron on the surface of the antenna rod. The electron is often stopped in its movement. It does not accelerate all the quarter period of the generator. Instead, it constantly collides with other electrons and is moved to free atomic trunks. So there is a lot of photon emission during each quarter period. The wave character of this EM radiation comes from the periodicity of the generator and the resulting summary movement of electrons in the same direction and from the polarisation of the electric and magnetic field components of the emitted photons. $\endgroup$ Commented Jun 10, 2022 at 4:23
  • $\begingroup$ Is charge carrier density an intrinsic property of a material and is thus constant? $\endgroup$ Commented Jun 13, 2022 at 3:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.