Why are they dots and not squares, triangles, or some other shape?

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Not-much- they just have to be closer together than the wavelength of the microwaves.

Generally the door has a metal grid, with holes in it smaller than the microwaves. This stops the radiation getting through but allows the much smaller wavelength visible light to pass through.

Either you are mis-understanding what you are seeing, ie the white dots are really holes in a dark mesh, or the maker has used a pattern of metal dots instead of a mesh.

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  • $\begingroup$ Also with circles there is no need for any implicit assumption on how polarization of the waveform. A square or triangle on the other hand will be more permeable for wave polarized along a certain plane, than others. $\endgroup$ – icarus74 Dec 16 '11 at 8:24

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