A microwave oven uses a device called a magnetron tube, which causes an electron beam to oscillate at very high frequencies, producing microwave (MW) radiation. Domestic and commercial units use a frequency of 2.45 gigahertz (GHz) at an output of 400-900 watts for a typical domestic oven, whose power supply is designed to deliver 4000 volt pulses to the magnetron. The 2.45-GHz frequency is used because water absorbs electromagnetic energy quickest and maximally at this frequency, thus allowing food containing water to be heated quickly.

The molecules within the food are forced to align themselves with the very rapidly alternating field and to oscillate around their axis. Heat is produced from the considerable intermolecular friction.Microwaves are beamed from the magnetron into the oven compartment, where they heat the food from the inside out, unlike conventional ovens, which do the reverse. Heating from the inside first can give rise to cold spots hence the need to rotate the dish constantly.

The maximum leakage level allowed under

current standards is a power density of 5 milliwatts per square centimetre at a distance of 5 centimetres from the oven door. This limit is based on standards for MW radiation that are disputed by those who argue that non thermal effects of MW radiation should be taken into account in tallying radiation levels (as, for example, with mobile phones). The door itself should be checked periodically to ensure that it is not leaking excessively.