Did you need the Nikola Tesla explanation, or the conventional physics approach? (The non-math versions, in each case?)

Conventional physics first.

A metal plate reflects EM waves, including radio and light. A polished metal plate is a good mirror. Even for all of the longer-than-optical wavelengths, it still behaves as a “polished mirror” for microwave and radio waves. But note that the mirror doesn’t guide any waves. Ideally, the waves come in, and are 100% reflected back out again. Angle of incidence equals angle of reflection. During reflection, patterns of electric currents appear in the mirror. The patterns are exactly those which produce a “transmitting antenna” to cancel out the incoming wave, and to generate the reflected wave, and send it off at the same angle as the angle of incidence. That’s basic mirror physics 101.

But is it possible to somehow attach the waves to the mirror? What if we send our beam at an extremely shallow, glancing angle? Will angle of incidence no longer equal the zero angle of reflection? Or, what if we use a slightly curved mirror, such as a wide hemisphere, then guide some waves tangentially across the edge? Will it stop being a reflecting mirror, and instead “grab” the waves and guide them along its surface?

The answer turns out to be “yes,” but it’s a feeble effect, and it requires that the mirror surface be resistive (and so, not a 100% reflector.)

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