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In our physics class, we were taught that radio waves would reflect from surfaces even if they have gaps. Example, radio telescopes are made from mesh instead of the solid, or a microwave that has a mesh on the door.

We were taught that for a wave to be reflected the gaps in the material must be smaller than 1/20th of the wave's wavelength. Our teacher said that he also does not understand where this number comes from but that it is quoted in all textbooks.

I searched the internet for an answer but was not able to find anything apart from "the gap should be significantly smaller than the wavelength"

Is there an explanation as to why it's 1/20th or is it just an arbitrary number?

Thanks in advance!

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    $\begingroup$ See physics.stackexchange.com/questions/149607/… I think it is a "rule of thumb", not any formal limit. Attenuation should go somethng like this ratio to the power of 4. $\endgroup$
    – ProfRob
    Commented Nov 26, 2021 at 13:50
  • $\begingroup$ I see, this is intersting, thanks! $\endgroup$
    – Eugene Levinson
    Commented Nov 27, 2021 at 15:37

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