Friday, December 30, 2016

EM wave , diffraction, minimum hole size

Good discussion here on topic of EM waves passing through holes of different sizes:

You need to distinguish between conductors and non-conductors. If your material is non-conducting then the EM radiation can pass through any size of hole regardless of whether the hole is larger or smaller than the wavelength. The power transmitted is just the incident power per unit are multiplied by the hole area, exactly as you would expect. All very boring really.
But you mention the screen in a microwave, and in this case the screen is conducting, which completely changes the behaviour. 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.
Conducting screens like the screen in a microwave are generically known as Faraday screens or more commonly Faraday cages. Calculating the relationship between the hole size and the transmitted intensity is a somewhat tortuous business, but as it happens there is an excellent description of the calculation here on thi site in David's answer to What is the relationship between Faraday cage mesh size and attenuation of cell phone reception signals?.

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