Why are the microwaves in a microwave oven tuned to water?

Public Domain Image, source: Christopher S. Baird

The microwaves in a microwave oven are not tuned to a resonant frequency of water. In fact, the microwaves generated inside a microwave oven are not really tuned to any particular resonant frequency since the waves are broadband. A broadband electromagnetic wave contains many frequencies. You need a monochromatic wave (a nearly single-frequency wave) in order to tune to a specific frequency. Laser beams are monochromatic. Radio waves from simple antennas are monochromatic. Microwaves in an microwave oven are not monochromatic.

The microwaves in a microwave oven are created by a device called a magnetron, which is a resonant cavity that causes current to naturally oscillate at high frequency, and thereby emit electromagnetic waves. The oscillation of the current in the magnetron is not caused by a delicately-controlled external circuit. Rather, the oscillation arises naturally from the way that electrons emitted by the cathode hit the anode in a random fashion and then slosh around as directed by the magnetron's shape. This randomness causes the magnetron to emit many frequencies. Furthermore, the random nature of the oscillation generation also causes the frequencies to be unstable and quickly jump around. A study of a typical household microwave oven conducted by Michal Soltysiak, Malgorzata Celuch, and Ulrich Erle, and published in IEEE's Microwave Symposium Digest, found that the oven's frequency spectrum contained several broad peaks that spanned from 2.40 to 2.50 GHz. Furthermore, they found that the location, shape, and even the number of broad peaks in the frequency spectrum depended on the orientation of the object that was in the oven being heated. In other words, the exact frequencies present in the electromagnetic waves that fill the oven depend on the details of the food itself. Clearly, the microwaves cannot be tuned in frequency to anything particular if the frequencies change every time you heat a different food. In applications where stable monochromaticity is important, such as in radar imaging, the magnetron therefore has limited utility. In applications where energy delivery is more important than monochromaticity, such as in a kitchen oven, the magnetron is ideal.

So how do microwaves in an oven heat food if they are not tuned to a specific resonant frequency of water? They heat the food through simple dielectric heating. In dielectric heating, the electric field in the electromagnetic wave exerts a force on the molecules in the food, causing them to rotate in order to align with the field. Because of this rotating motion, the molecules collide into each other and convert their somewhat ordered rotational motion into disordered motion, which we macroscopically call heat. Many types of molecules in the food absorb energy from the microwaves in this way, and not just water molecules.

The book Electromagnetics Explained, by Ron Schmitt states: