Microwave ovens use microwaves (duh) to heat food. Microwaves are part of the Electromagnetic Spectrum – relatively long wavelengths of particle-like radiation moving at the speed of light. Microwaves are shorter wavelengths than radio waves but longer wavelengths than infrared.
Even though microwave radiation has less energy than infrared, visible light and other types of electromagnetic radiation further up the spectrum, there is still plenty of energy in microwaves to “zap” our food. Microwaves in the 2.5 gigahertz range are used in microwave ovens. In this frequency range microwaves have an interesting property: they are absorbed by water, fats and sugars. When they are absorbed they are converted directly into atomic motion — heat. Microwaves in this frequency range have another interesting property: they are not absorbed by most plastics, glass or ceramics. Metal reflects microwaves, which is why metal pans do not work well in a microwave oven.
You often hear that microwave ovens cook food “from the inside out.” How does this work compared with a regular, conventional oven? In a conventional oven, the heat has to migrate (by conduction) from the outside of the food toward the middle. You also have dry, hot air on the outside of the food evaporating moisture. So the outside can be crispy and brown (for example, bread forms a crust) while the inside is moist. In microwave cooking, the microwaves penetrate the food and excite water and fat molecules pretty much evenly throughout the food. No heat has to migrate toward the interior by conduction. There is heat everywhere all at once because the molecules are all excited together. There are limits, of course. Microwaves penetrate unevenly in thick pieces of food (they don’t make it all the way to the middle), and there are also “hot spots” caused by wave interference, but you get the idea. The whole heating process is different because you are “exciting atoms” rather than “conducting heat.”
In a microwave oven, the air in the oven is at room temperature, so there is no way to form a crust. That is why microwavable pastries sometimes come with a little sleeve made out of foil and cardboard. You put the food in the sleeve and then microwave it. The sleeve reacts to microwave energy by becoming very hot. This exterior heat lets the crust become crispy as it would in a conventional oven.
You can “see” the food cooking in microwaves without being “zapped” because the metal screen through which you see reflects the microwaves. The holes in the screen are smaller than the wavelengths of the microwaves so the microwaves are reflected back into the microwave oven instead of passing through the glass of the oven.