The microwave is a form of electromagnetic radiation that is used in radar, communication, and cooking. It has a wavelength of about 10 micrometers (0.001-0.03 m). The radiation has a much shorter wavelength than radio waves and infrared light, but longer than visible light. It can penetrate food and other materials, and heat them up.
The basic technology behind the microwave oven was developed in the United Kingdom during World War II using radar technology, and is generally credited to American electrical engineer Percy Spencer. The first microwave oven was large and expensive, but rapidly reduced in size and cost after the end of the war. By the late 1970s, countertop microwave ovens were available from most major appliance manufacturers.
A microwave oven cooks food by creating a circular pattern of loops of high-frequency radio waves that oscillate with the frequency of water molecules in the foods being heated. The jiggling of the water molecules creates friction that heats up everything the water comes in contact with.
One of the reasons it’s safe to eat foods cooked in a microwave is that the heat doesn’t typically exceed the boiling point of water. However, it is possible for fatty foods to become hotter than they should be when heated in a microwave. Microwave ovens are also generally safe to use for heating liquids, but it is important to stir them and use a lid to prevent splattering.