933. What metals and other substances are used in microwave ovens? Specifically, what is the substance on the inside of the microwave that absorbs all the microwaves? — AD, San Anselmo, CA
The walls of a microwave oven's cooking chamber are made of highly conductive metals so that they reflect the microwaves almost completely. Only a very small fraction of the microwaves inside the oven are absorbed by these metal walls and virtually none of the microwaves escape into the room. However, there is a substance inside the cooking chamber that absorbs the microwaves: water in the food! If you don't put water-containing food inside the microwave oven, there will be nothing to absorb the microwaves and they will reflect back to the magnetron and may damage it. The absence of an absorber in the cooking chamber will also increase any minor leakage of microwaves from the oven because the microwave intensity inside the cooking chamber will be much higher than normal.