Percy Spencer is associated with the first practical microwave oven after discovering microwave heating in 1945.
Spencer was an engineer at Raytheon working with magnetrons, the vacuum tubes that generate microwaves for radar. In 1945, he noticed that radiation from an active magnetron melted a candy bar in his pocket. He tested the effect with other foods, including popcorn and an egg.
Raytheon developed the discovery into the Radarange, the first commercially available microwave oven. The earliest models were large, heavy, and expensive, standing roughly six feet tall and weighing hundreds of kilograms. Later improvements made countertop microwave ovens practical for homes.
Microwave ovens do not cook food through ordinary external heat. Their electromagnetic waves cause polar molecules, especially water molecules, to move and generate heat. The metal interior reflects microwaves, while the rotating turntable helps expose food more evenly.