The cavity magnetron is the heart of a microwave oven. It’s pretty much the same gadget invented during the Second World War to boost the radar abilities of the Allied Forces against the Germans. The cavity magnetron uses electric current and a strong magnetic field to send microwaves into the oven’s cooking area.
Once there, the microwaves bounce off reflective metal walls (often covered in plastic). If there’s no food in the oven, the microwaves may bounce back and hit the magnetron, shortening its life.

When microwaves meet food, they heat the food’s water molecules. Since microwaves affect only liquid water, moist foods (like chocolate) cook better than dry foods. So, while microwaves are good at heating liquid water, they’re not good at heating ice. The defrost setting on microwaves, in which the magnetron repeatedly kicks on and off, allows frozen food to melt a little between on cycles. This increases the liquid water target for the microwaves.

Why can’t you put metal in a microwave oven? Since the walls of a microwave oven are metal, any other metal object (like a fork) can cause sparks to jump between itself and the metal oven walls, damaging the oven or even starting a fire.

Finally, why do people say nuke it when referring to microwave ovens? Microwave ovens do use radiation to cook, but this radiation has nothing to do with the radiation from nuclear energy. The radiation from nuclear energy can be dangerous because each bit of that radiation might kill cells. Any one bit of microwave radiation, on the other hand, is no more dangerous than any one bit of visible light. It’s the ability of the magnetron to concentrate lots of microwave radiation in a small space that makes microwave cooking possible.

From a jumpy compass to a war-time weapon to a perfect chocolate chip cookie warmer — quite a journey for an invisible bit of light!

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