A microwave oven uses microwaves to heat food. Microwaves are radio waves. In the case of microwave ovens, the commonly used radio wave frequency is roughly 2,500 megahertz (2.5 gigahertz). Radio waves in this frequency range have an interesting property: they are absorbed by water, fats and sugars.
When they are absorbed the alternating electromagnetic field generated inside the microwave oven would lead to excitation, rotation/collision of polar molecules and ions inside the food. These molecular frictions would generate heat and subsequently lead to temperature rise.
Microwaves in this frequency range have another interesting property: they are not absorbed by most plastics, glass or ceramics.
The propagation mode of microwave and light is very similar, so microwave has a strong light transfer characteristic. They are all spread in a straight line. And when the microwave encountered an object which has a large size of its wavelength, it will be reflected. And metal reflects microwaves, which is why metals cause spark in a microwave oven.
In microwave cooking, the radio waves penetrate the food and excite water and fat molecules pretty much evenly throughout the food. There is no “heat having to migrate toward the interior by conduction”. There is heat everywhere all at once because the molecules are all excited together.
Principles of microwaving
