How exactly do microwave ovens work? I’d like to understand the science behind how they heat food but not the dishes..

-Karthik Gurumurthy

Microwave ovens are actually radar sets. Radar was developed before World War II in both England and the United States and played a crucial role in giving the British a fighting chance in the Battle of Britain in 1940-41. The early radar workers sometimes made an interesting discovery – when radar technicians moved in front of a working radar antenna, the candy bars in their pockets would melt.

The heart of a microwave oven is a fist-sized vacuum tube called the magnetron. This electronic device creates electromagnetic waves by using electricity to heat a filament wire. The resulting electrons wiggle and emit waves at about 2450 MHz. Microwaves are essentially the same as light waves, but you can’t see them. Each wave measures about five inches in length, too long for the human eye to detect. Inside the microwave oven, a fan distributes these waves evenly throughout the cooking chamber.

Most foods requiring cooking or heating contain significant water. Water molecules consist of bipolar hydrogen and oxygen atoms – the oxygen atom is slightly negative while the hydrogen atom is slightly positive. When microwave radiation strikes these water molecules, it causes them to vibrate rapidly and rotate back and forth, switching directions constantly. This rotation occurs millions of times each second, and all this twisting movement creates friction that heats the food.

The story is different for dishes, though. Dishes contain very few water molecules, so there are practically no water molecules to be affected by the microwaves. That’s why the dishes themselves generally don’t heat up in a microwave.