Microwaves are electromagnetic waves that can transmit thermal energy. They can travel through empty space and also through food or other materials, although they are absorbed by some substances, such as water vapor in clouds or the liquid inside a microwave oven. Microwaves have the same properties as other radio waves, but they have a higher frequency. This allows them to penetrate materials more deeply than lower-frequency radio waves, such as AM or FM broadcasts.
Microwave ovens use microwave radiation to heat foods quickly. The radiation is absorbed by the molecules in the food, which causes them to vibrate and give off heat. Spherical foods cook more quickly than irregularly shaped foods, because the microwave energy has a shorter distance to travel through them. Microwave ovens contain a metal cavity that the food is cooked in, and an oscillator that generates microwave radiation. The oven’s magnetron creates microwaves by magnetically exciting electrons in the cavity.
Various types of vacuum-tube devices can produce microwaves, including the klystron and the cavity magnetron. Solid-state devices like the Gunn diode and tunnel diode can also generate microwaves.
The microwave oven was invented by Percy Spencer, an engineer who worked on radar research before World War II. He was playing around with a magnetron at Raytheon and accidentally melted a candy bar and an egg in the device’s experimental chamber. Spencer’s experiment inspired him to build a working microwave oven, which was patented in 1946. Microwave transmissions carry all kinds of information, from analog and digital voice, data and video to supervisory control and data acquisition (SCADA) for remote machinery and signals. Cell phone towers use microwave transmitters to relay wireless calls, and microwave antennae are used in police radar guns for tracking weather and air traffic control. Scientists have also discovered cosmic microwave background radiation, which fills the Universe and is one of our first clues about the Big Bang.