Not surprisingly, a microwave oven uses microwaves to heat food. Microwaves are a type of wave sandwiched between radio waves and infrared radiation on the electromagnetic spectrum. In the case of microwave ovens, the commonly used wave frequency is roughly 2,450 megahertz (2.45 gigahertz).
Waves in this frequency range have an interesting property: Water, fats and sugars absorb them. Once absorbed, they get converted directly into atomic motion — heat. These waves boast another interesting, related property, too: They're not absorbed by most plastics, glass or ceramics. Metal reflects microwaves, which is why metal pans don't work well in a microwave oven. It's also why the devices have metal walls — for reflection.