How a microwave oven heats food, and why the middle stays cold
What a microwave oven really does to the water in your food
In 1945, a chocolate bar melted in an engineer's pocket, with no stove or flame anywhere near him. That accident gave us the microwave oven. Here is what its invisible waves do to your food, and why they hardly warm the air inside.
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In 1945, an engineer named Percy Spencer was working next to a tube that made waves for radar. He reached into his pocket, and his chocolate bar had melted. There was no stove or flame anywhere near him.
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His company soon built the first microwave oven around that kind of tube. Your oven at home has one too, and it is called a magnetron. Tonight you use it to heat yesterday's soup.
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You press start. The magnetron fills the metal box with microwaves, a kind of radio wave. They bounce off the metal walls and pass through the soup again and again.
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Soup is mostly water. Zoom in on one drop, and you see water molecules, the smallest pieces of water. Each one has a slightly negative end and a slightly positive end.
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The microwaves pull on those two ends, first one way, then the other. The waves swing back and forth 2.45 billion times every second, and each molecule keeps turning to follow them.
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Turning molecules knock into the ones next to them, and those start to move faster too. All that jostling is heat. Air has almost no water in it, so the waves pass through it and hardly warm it.
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The soup is ready in two minutes. Then Rono, your flatmate, puts in a thick slice of cold lasagne from the fridge, and he sets the timer for 3 minutes.
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Quick check
Two paper towels.
Show the answer
The wet one. Its water molecules turn with the waves and heat up. Dry paper has almost no water for the waves to shake.
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Go further
Three names for what you saw.