A microwave oven is an excellent example of a device in which several fundamental concepts about waves find practical application.

It was invented "by accident" in 1945 during the development of microwave generators for radar. The first commercial oven was the size of a large modern refrigerator and on its own drew all the power that is available today to an apartment.

What are microwaves

Microwaves are electromagnetic waves with wavelengths roughly between 1 mm and 30 cm. Their wavelength is therefore shorter than that of radio waves, but longer than that of infrared rays.

Since light travels at about 300 million m s^-1, these wavelengths correspond to frequencies between 300 billion and 1 billion hertz respectively. One billion hertz is called a gigahertz, symbol GHz.

Besides ovens, microwaves are used above all in radar and in telecommunications. They are used, for example, by GSM mobile phones, satellite TV, and wireless computer networks such as Bluetooth and IEEE 802.11.

The parts of the oven

A microwave oven essentially consists of:

1. a microwave generator called a magnetron
2. a microwave waveguide
3. a cooking cavity

as well as power supply and control circuitry, which does not concern us here.

The magnetron is a high-power vacuum tube that generates microwaves with a frequency of 2.45 GHz, corresponding to a wavelength of 12.24 cm, with a power of about 700 W. Another 400 W are lost as heat.

The waveguide is simply a metal tube that carries the microwaves into the oven, preventing them from spreading out (for details on how it works see the section waveguides).

The cooking cavity is a Faraday cage, that is, a cavity enclosed by a metal mesh. The mesh has openings much smaller than 12 cm (the wavelength of the microwaves), and therefore reflects the waves inside (on this, see the page reflection), preventing them from leaving the oven. Standing waves are set up inside it almost immediately. Consequently there are points inside the oven where the field intensity is always at its maximum (antinodes), and others where it is always at its minimum (in theory zero: the nodes). This is why the plate holding the food to be cooked usually rotates.

Why does food heat up?

The oven actually heats mainly the water contained in food.

The heating itself does not occur by resonance, but by "dielectric heating". Here is how it happens.

Water molecules have an "electric dipole", that is, a pole of positive charge and one of negative charge. A dipole placed in a field tends to align with the field. But the microwave field oscillates, reversing its direction 2.45 billion times per second. This forces the water molecules into very rapid motion. As they move, the water molecules collide with other molecules and pass on part of their energy.

Recall that the temperature of a body is proportional to the average kinetic energy of its molecules. So, after a while, part of the energy of the microwaves is transferred to the molecules of the food, which becomes hotter.

Remarks

• fat and sugar molecules are much less polar than water, so microwaves cannot easily set them into strong agitation. They are heated indirectly, as explained above, through collisions with the hotter water molecules.
• frozen water molecules are much less mobile than in the liquid state. So defrosting food is harder than heating it once defrosted. This is why ovens usually have a special "defrost" function.

So does my mobile phone also heat my ear while I talk?

Ultimately yes, but the amount of heating is not comparable with that of an oven, for several reasons:

1. a microwave oven usually has a power of about 1000 or 1500 watts, while a mobile phone emits an average power of 0.25 watts (the peak value is about 1 watt).
2. the oven concentrates all the microwave power through the resonant cooking chamber, while a phone spreads its already small power throughout the surroundings, and in particular towards the base station it communicates with.