Microwave cooking has been around for more than forty years. In 1945, Dr. Percy Spenser patented a microwave that he built from a milk can. The first ovens for home use were first marketed by Raytheon in 1955.
Microwaves are a form of magnetic and electrical energy moving through space.
They are absorbed by foods but reflected by metal. The microwaves, which are produced by a magnetron element, bounce around inside the metal oven until they are absorbed by food, and produce heat. Although the microwaves produce heat directly in the food, it is uneven. The microwaves do not cook food from the inside out, especially thick foods such as roasts. They generally cook about an inch of the outer layers.