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This answer describes why EM waves from inside a microwave are blocked by the metal mesh screen:

The incident EM wave induces oscillations in the metal of the screen, and these oscillations reradiate EM that interferes with the incident wave. It's this process that blocks the incident wave. The process is purely classical and requires no appeal to the existance of photons.

and refers to another answer for a calculation (Which is based on setting up a standing (and partially traveling) wave equation and solving for boundary conditions representing the screen waveguide. It's then shown from it that low frequency EM waves will exponentially decay with distance from the screen. This is my superficial understanding of it, so my summary might be wrong)

I have two questions on this:

  1. The incident EM wave oscillates electrons, so there must be some delay (as these electrons oscillate) which seems like it'd be manifested as a phase difference between the reradiated EM wave and the incident one. Also, the electrons can only oscillate along the plane of the metal screen so the reradiated wave caused by such oscillations would seem to be produced at some angle to the incident wave. If so, how does the reradiated wave exactly cancel out the incident wave, if it's neither in phase nor in the same direction as it?

  2. The reradiated wave would seem to go in all directions, including backwards into the microwave. So why doesn't it also interfere with (or cancel) the incident wave there?

(Since this likely has a classical explanation, I would appreciate if possible an answer that uses classical electromagnetism).

user981981
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  • If studying ocean waves you do not worry about how each individual atoms might be flowing with the wind, why do you care what the reaction of the electrons might be to the incident EM wave at the metal surface unless you are interested in, say, the photoelectric effect, or in compaction, etc.? The radiation formula of Lienard-Wiechert is no less mysterious than the phenomenological antenna and scattering theory of Hertz, Ignatowski, Stratton, etc., derived straight from the macroscopic Maxwell's equations. – hyportnex Sep 15 '23 at 19:08

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EM waves (and all waves) are never truly cancelled, they can superimpose but that is a temporary phenomenon. A wave in a medium is an excitation of the medium but the medium can never absorb or cancel the energy .... energy is only removed from the medium by absorption ... i.e water waves crossing on the shore.

In the microwave the metal screen mostly reflects the waves (but there is a small amount of absorption as well). In the classical world (historically based) the reflection is typically out of phase with the incident radiation and this leads to the conclusion of waves cancelling ... but it is incorrect. 2 perfectly align EM field waves that are out of phase have net zero E and M fields .... but the energy in the EM field is not zero.

PhysicsDave
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