Physics of the microwave oven

This is the first of two articles about the physics of microwave ovens. This article deals with the generation of microwaves in the oven and includes the operation of the magnetrons, waveguides and standing waves in resonant cavities. It then considers the absorption of microwaves by foods, discussing the dielectric relaxation of water, penetration depths of electromagnetic waves in matter and, in considering the possible chemical changes during the microwave heating, multi-photon ionization or dissociation. What are microwaves? Microwaves are electromagnetic waves. Their frequencies (wavelengths) are in the range from 300 MHz (λ = 1 m) up to 300 GHz (λ = 1 mm). Regarding wavelengths as typical spatial dimensions, one realizes that microwaves do not have dimensions ofμm, as might be expected from the misleading ‘micro’ in their name. Following international conventions, microwave ovens at home or in restaurants operate at frequencies of about 2.45 GHz, i.e. λ = 12.23 cm. Physics of a microwave oven Figure 1 depicts a typical microwave oven (many details can be found in [1–3]). Microwaves are generated in a magnetron which feeds via a waveguide into the cooking chamber. This cuboid chamber has metallic walls and so acts as a Faraday cage. The front door, made of glass, and the light bulb cavity are both covered by metal grids. The holes in the grids are small compared with the wavelength of the microwaves, hence the grids act just like metal plates. Most microwaves cook the food on a rotating turntable in this chamber, but some designs include a rotating reflector, acting as a stirrer. Expensive models may include thermometers, additional 1. Microwave oven