Inside the oven a wave is trapped in a metal box and heats in spots. Switch the spinning off, press “Start” and look at the food through the thermal camera. Then measure the speed of light with an ordinary chocolate bar.
The magnetron makes a radio wave that oscillates 2.45 billion times a second. A waveguide carries it into the cavity, and the metal walls bounce it back and forth. The direct and reflected waves add up: where the crests line up the field is strong, where they cancel it is almost zero.
The result is a standing wave: the hot spots stay put. Turn on “Show the wave” to see them right above the food.
Neighbouring hot spots are half a wavelength apart — about 6 cm.
The spots don’t move, so without spinning some bites boil while their neighbours stay cold. The plate carries every bite in a circle through hot and cold places, and the heating evens out.
It’s never perfectly even: a point near the edge rides its own circle, a point near the centre another. That’s why rings remain. Expensive ovens use a spinning metal “stirrer” instead of a plate, which mixes the wave itself.
Switch to “Chocolate”: the plate is removed so the chocolate doesn’t move. Press “Start”. The chocolate melts in spots. Then tap the centres of two neighbouring spots and the oven works out the speed of light.
Try it at home: a chocolate bar, no plate, 15–20 seconds, a ruler. The frequency is on the sticker at the back of the oven.
Its holes are about 2 mm, while the wave is 12 cm long. A wave that long can’t get through tiny holes, but light can. So you can see the food, and the radiation stays inside.
At sharp metal edges the field concentrates in one point and gets strong enough to break down the air. Smooth, thick metal dishes usually don’t spark — but they shield the food.
2.45 GHz is not a resonance of water. Water molecules just try to turn with the field and rub against each other. The frequency was chosen because it’s allowed for household devices and heats to a convenient depth.
The microwave’s sticker says 2,450 MHz. The plate was removed and a chocolate bar was heated. How far apart will the centres of neighbouring melted spots be? The speed of light is 3·10⁸ m/s.