Visible light is just EM radiation at a specific frequency, and mirrors do a pretty good job reflecting that. Is it possible to make a 'mirror' that reflects radiation below 1MHz, 1kHz, or even 100Hz?
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Practically any metal surface is a reasonably good mirror. To lighten their weight one can even apply a sparse grid instead of a continuous surface. The spacing of the wire grid should be less than $1/2$ of the wavelength.
A very common domestic example is your microwave oven whose front window is transparent to your eyes but there is a metallic grid printed on it with a few mm-s spacing to reflect the 12cm radiation. Of course, the denser the grid is the less it leaks. For all practical EM purposes the microwave oven acts as a closed cavity whose walls are mirrors at the operating frequency ~2.5GHz.
At lower frequencies in the MHz, KHz range the spacing can be proportionally larger and such mirrors are buried in the soil or built around most monopole antennas for LH and HF transmissions to act as the electric "ground" to the pole.
answered Oct 22, 2022 at 14:07