What would happen if one were to do the following?
[li]prepare a glass mirror, to fit exactly inside the floor of a microwave oven[/li][li]remove original microwave tray, place mirror on floor of microwave with the reflective side facing up[/li][li]place something, for example, a bag of microwave popcorn, on the top of the mirror[/li][li]turn on the power[/li]
I W.A.G. that this would increase the efficiency and make the popcorn pop faster and/or more evenly. I base this on nothing but pure speculation, however.
My EE WAG is that nothing will change. IIRC, the floor and walls of a microwave oven are already reflective to the microwave radiation (otherwise they’d heat up and absorb some energy, instead of bouncing it back to the food). Puting another reflector over the reflective floor of the microwave shouldn’t change anything.
The typical microwave is constructed inside and out with metal. The microwave chamber is metal.
Metal is not the culprit in a microwave oven. Creating a circuit with a loop or circle of metal is what you want to avoid. The only thing worse than a circuit of metal in the microwave, is ALMOST a circuit - the induced electricity will arc to complete the circuit - possibly causing sparks and flames.