How come we can put aluminum foil in the oven but not microwave

paul84 · 36-40, M

An oven is just radiant heat from gas or an element, so it just... heats things up like any other source of heat. So no special properties there.

Microwaves use radio waves. When the radio waves hit food they are conducted around an inch in and heat up the food. Thick metal reflects radio waves, but thin metal isn't strong enough to reflect, and so instead absorb them and heat up, eventually catching on fire. As well, there's an electromagnetic field in a microwave, and thin metal will conduct that and shoot off sparks.

RealtaReoite · 61-69, M

In a microwave, foil, especially crinkled up foil, messes up the electric fields produced by the short wavelength radiation. High currents can flow in the foil even to the point where it catches fire. It also reflects a lot of the energy so food wrapped in foil doesn't get hot.

Infrared radiation in a conventional oven doesn't care one way or another about foil. Well, ok, it reflects some, but nothing interesting happens.