Microwave is a form of electromagnetic radiation, with wavelengths shorter than those of radio waves and infrared radiation but longer than those of ultraviolet (UV) and X-ray (which smash into molecules, causing them to vibrate and heat up). Microwaves are used for industrial heating, drying, curing, and sterilizing processes. They are also used for telecommunications applications such as telephony, fax, and cellular data transfer. Radio astronomers use microwaves to observe extraterrestrial radiation, but due to attenuation by the atmosphere they mostly observe it from high-altitude balloons or satellites.
In a microwave oven, a magnetron produces electromagnetic radiation at microwave frequencies (about 2.45 GHz). The waves are carried through a waveguide to a cooking cavity where they enter the food. The water, sugar, and fat molecules in the food absorb these rays and start to heat up. The friction between these molecules generates the heat that cooks the food (the molecules in the food get hot, but the air around the food does not).
A cooling fan, usually located on the bottom of the appliance, turns on and off to keep the cooking cavity cool and prevent odors from building up inside the microwave. Most microwave ovens feature a variety of power settings, such as defrost or high-heat. When a power setting is selected, the magnetron emits microwaves for a specified amount of time at a set frequency. To control the duration of the microwave emission, the oven may be programmed to turn it on and off at intervals, such as two seconds on and five seconds off.