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How Microwaves Cook Food. When you hit the start button it usually takes about 2 seconds to heat up a filament inside the magnetron tube, says Schiffmann. The microwaves are then blasted into the food compartment. The commonly used frequency of microwaves, 2.45 gigahertz, is easily absorbed by water, fat, and sugar.
The noise is from the fan that blows air across the magnetron to keep it cool. Microwave ovens also produce a hum. It comes from the transformer, diode and capacitor, which vibrate as they step up the 60-hertz electric power from a wall outlet. SPARKS: Despite common wisdom, metal does not necessarily cause sparking inside a microwave; indeed, the cooking chamber walls are metal.