EM waves have an amplitude and a frequency. The atoms in a frequency selective surface are placed such that a wave of particular amplitude and frequency is guaranteed to run into and be deflected by the surface. Since all radiation is traveling at the speed of light, this variable doesn’t have to be accounted for.
For a simple example, look at microwave oven doors: EM radiation in the visible spectrum can get through the holes, but microwaves are the right size to not fit through the holes because of their amplitude and frequency of oscillation while traveling at the speed of light.
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