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The following question was proposed by a student durante a lecture for a grad course in EM.

What happens to the energy of an EM wave during destructive interference in vacuum, in regards to the conservation of energy?

Interestingly, for some reason I've never gave the proper thought about it, and while we indeed arrived at a solution, I thought it would be interesting to share it here due to the conceptual importance to undertanding the nature of waves.

I'll post the answer later in the day, so in the meanwhile feel free to offer your answers or interpretations regarding this question (note that there's a similar question on the site but it only deals with material media, so please don't flag this)

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Whenever there is destructive interference (e.g., at one exit port of a Michaelson interferometer), there is constructive interference (e.g., at the other exit port of a Michaelson interferometer).

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    $\begingroup$ yeah but a constructive interf has 2x amplitude and therefore 4x energy, so the energy averaged up for 1 constructive (4x energy) and one destructive (0 energy) would be 2 times the energy of the wave $\endgroup$
    – Manu de Hanoi
    Commented Apr 1, 2019 at 16:44
  • $\begingroup$ And what do you think is the meaning of the result of this "average" between the minimum and the maximum values? $\endgroup$
    – nasu
    Commented Apr 1, 2019 at 21:00

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