The cooking chamber of a microwave oven is always metallic. Even the glass door has a metal grid across it to keep the microwaves inside. This metal chamber may be coated with paint or plastic but it is there nonetheless. Without it, the microwaves would leak out and the oven would be hazardous and inefficient. It would cook objects throughout the kitchen.
The electric field of a microwave flips back and forth at just about the right frequency to have the largest effect on water molecules. The water molecules try to follow the reversing electric field and, in doing so, become hotter and hotter. Radio waves flip too slowly to have very much effect on water. Furthermore, the microwaves in an oven are far more intense than the radio waves that we’re used to have around us so that common radio waves just don’t do very much cooking.
As noted previously, the water molecules in frozen foods are not all bound up perfectly inside ice crystals. As long as there are a few relatively mobile water molecules, even frozen food will eventually absorb enough energy to melt. Once that happens, the food can cook easily. Of course, the melting process is frequently very non-uniform so that food comes out with hot and cold regions. In general, frozen food cooked in a microwave is not very satisfying.
If you arrange a metal rod so that it’s parallel to a microwave’s electric field, the microwave will push electric charges up and down that rod. This moving charge will waste some of the microwave’s energy by creating heat in the rod. But the main effect will be that the rod will reflect or scatter the microwave. The moving charge will emit its own microwave and this new microwave will interfere with the original one.
In any frozen food, there are some water molecules that are relatively free to turn about. These molecules may be at the surfaces of ice crystals or sitting on the surface of food particles. These water molecules can absorb microwaves and heat. However, the heating is very uneven because as soon as any water crystal absorbs enough heat to melt, the resulting liquid water will begin to absorb microwaves much more strongly. That is why defrosting must be done slowly. Then the microwave deposited heat will have time to flow through the food and melt it uniformly. Otherwise, you can end up with boiling hot spots mixed together with frozen icy spots.
When it’s active, the magnetron’s cavity has electric charge sloshing back and forth along its tines. The charge moves at a frequency determined by the shape and size of the cavity and these are carefully controlled so that the cavity’s natural resonance frequency is 2.45 gigahertz. To keep the charge sloshing, the magnetron adds negative charge from a hot filament wire located in the center of the cavity. Electrons flowing off of this wire are steered toward the negative tines by a magnetic field. As a result, the charges continue to slosh back and forth indefinitely. A small wire connected inside the magnetron extracts some of the energy in the magnetron and converts it into microwaves outside the magnetron. This wire acts as an antenna. The antenna is located in the pipe that carries the microwaves to the cooking chamber.
Both microwave ovens and car phones emit electromagnetic radiation. But that radiation has relatively long wavelengths (about 12 cm in the case of microwave ovens and about 40 cm in the case of car phones) and is not at all like the electromagnetic waves emitted by nuclear processes. Nuclear electromagnetic radiation, usually called gamma rays, has extremely short wavelengths (less than 0.001 nanometer or about a millionth of the wavelength of visible light). All electromagnetic waves are emitted and absorbed as particles called photons. The energy in a photon is inversely proportional to its wavelength (in vacuum). Gamma rays, with their short wavelengths, have very energetic photons that can do lots of chemical damage to your tissue. But the longer wavelength radiation from microwave ovens and car phones comes as very low energy photons. These photons can’t do chemical damage. The only thing those waves can do is heat things. Microwave ovens are carefully shielded so that they keep most of the microwaves inside. If those waves did emerge, they would simply warm your tissue up. This warming won’t cause genetic damage but it could cook your tissue. There has been recent concern about low frequency electromagnetic fields causing subtle damage to tissue, but these have not be substantiated by scientific research and no physically reasonable scenarios for how such damage could occur have been offered.
It’s doubtful that microwave cooking forms free radicals in food. The microwaves in a microwave oven cook by exerting torques on the water molecules and gradually increasing the water molecules’ thermal energies through friction-like effects. There is never enough energy present in a single molecule at one time to shatter that molecule and form a free radical. While ultraviolet light, such as that found in sunlight, carries enough energy per photon (particle of light) to split a molecule and form a free radical, microwave radiation carries very little energy per photon. That’s why microwave photons can’t do chemical damage the way ultraviolet photons can. However, even if microwave radiation could form free radicals in food, that wouldn’t necessarily cause you trouble when you eat that food. So much happens to the food before it enters your blood stream that a free radical probably won’t survive. The more harmful free radicals are ones that are actually created inside your body, where they can immediately attack important molecules in your cells.
I’ve already noted the issues of warming frozen food. However, the “defrost” setting is an interesting issue. If you’ve ever watched a microwave trying to defrost food, you’ve probably noticed that it heats the food briefly and then waits. It repeats this process many times. What it is doing is depositing energy (via the microwaves) into whatever water molecules are able to absorb microwaves. It then waits for this energy to flow as heat into the nearby food. Once the heat has been distributed rather evenly, the oven adds some more energy by turning the magnetron back on. This cycle of heating and waiting allows the food to defrost fairly evenly. Still, microwaves are likely to create hot and cold regions in the food so that some parts of the food will cook rather than defrost while some parts remain frozen.
If some radio station were to operate at 2.45 gigahertz, the main effect would be very poor reception of that channel on your radio. The oven isn’t a transmitter for microwaves; it just makes them like crazy. Most of the microwaves never leave the cooking chamber and there are strict regulations on any leakage. But it would only take a few thousandths of a watt of leaking microwave power to cause trouble in your reception of the radio station. Your radio wouldn’t be able to distinguish that station’s transmission from microwaves leaking out of your oven. The radio would struggle to pick up the signal and you would probably hear lots of noise in the background.