Microwaves, short, high-frequency radio waves lying roughly between very-high-frequency (infrared) waves and conventional radio waves (see  Electromagnetic Radiation). Microwaves thus range in length from about 1 mm to 30 cm (about 0.04 to 12 in). They are generated in special electron tubes, such as the klystron and the magnetron, with built-in resonators to control the frequency or by special oscillators or solid-state devices. Microwaves have many applications: in radio and television, radar, meteorology, satellite communications, distance measuring, and research into the properties of matter. Microwave ovens operate by agitating the water molecules in the food, causing them to vibrate, which produces heat. The microwaves enter through openings in the top of the cooking cavity, where a stirrer scatters them evenly throughout the oven. They are unable to enter a metal container to heat food, but they can pass through nonmetal containers.

Microwaves can be detected by an instrument consisting of a silicon-diode rectifier connected to an amplifier, and a recording or display device. Exposure to microwaves is dangerous mainly when high densities of microwave radiation are involved, as with masers. They can cause burns, cataracts, damage to the nervous system, and sterility. The possible danger of long-term exposure to low-level microwaves is not yet well known. Nevertheless, the U.S. government limits the exposure level, in general, to 10 milliwatts per square centimeter. Stricter limits are placed on microwave ovens.