How does a microwave oven work?

Open up a microwave and most of it is empty metal box. The food sits in the cavity, a box of painted steel. Metal walls reflect microwaves, so the waves bounce around inside until the food soaks them up. A glass turntable slowly turns the food.

The busy part is the column on the right. A heavy transformer turns the 230 V from the wall into about 2,000 V. A capacitor and a diode double that to about 4,000 V. This powers the magnetron, a special vacuum tube that turns electricity into microwaves. A metal duct called the waveguide carries them into the cavity. A fan keeps the magnetron cool.

The door has a metal sheet full of tiny holes, and interlock switches cut the power the moment you open it. A little control board runs the timer and the power level.

The magnetron is a vacuum tube the size of a fist. In the middle is a cathode, a coil of wire heated until it glows and boils off electrons. Around it is a thick copper anode, with about 4,000 volts pulling the electrons outwards.

On their own, the electrons would fly straight across. But two strong magnets above and below bend their paths into curves, so they whirl around the cathode instead.

The anode is carved with ten little cavities. Like blowing across a bottle makes it whistle, the swirling electrons make each cavity ring, with electric charge sloshing back and forth 2.45 billion times a second. The ringing bunches the electrons into turning spokes, which feed the cavities more energy. An antenna carries the energy off into the waveguide as microwaves, about 12.2 cm long.

It only works in a sweet spot. Too weak a magnet and the electrons crash straight into the anode. Too little voltage and they never reach it.

A water molecule is lopsided. The oxygen end carries a little negative charge and the two hydrogen ends a little positive charge. That makes it a tiny electric dipole, and an electric field tries to twist it into line.

A microwave is an electric field that flips direction 2.45 billion times a second. The water molecules try to keep turning with it, but they keep bumping into their neighbours, which slows them down. All that jostling is heat. This is called dielectric heating.

Ice is different. Its molecules are locked in a crystal and can hardly turn, so ice soaks up thousands of times less energy than liquid water. That's why frozen food thaws so unevenly: any spot that melts starts heating fast.

Microwaves don't cook "from the inside out". In wet food they are mostly used up in the outer 1 to 2 cm. The middle warms more slowly as heat conducts inwards, which is why recipes say to let food stand.

Inside the metal box, microwaves bounce off every wall. The waves going one way and the waves coming back add up into a pattern that stays in one place: a standing wave. Where they add, the field is strong and food gets hot. Where they cancel, at the nodes, it stays cold.

The hot spots sit about half a wavelength apart: 12.2 cm ÷ 2 ≈ 6 cm. That is why a microwave without a turntable leaves cold patches in your food. The turntable carries every bit of the plate through hot and cold places in turn, so it evens out. (Some ovens use a spinning metal fan, a mode stirrer, to stir the pattern instead.)

You can even measure the speed of light with it. Take out the turntable, heat a bar of chocolate for a few seconds, and measure the gap between melted spots. Speed = 2 × gap × frequency.

You can see through the door, but microwaves can't get out. The window has a metal sheet full of holes about 1 to 2 mm across. A microwave is about 12 cm long, far too big to squeeze through holes that small, so the sheet reflects it like solid metal. Light waves are less than a thousandth of a millimetre long, so they pass straight through.

If you open the door, at least two interlock switches cut the power before it opens, so the magnetron stops.

Metal inside is a different story. A flat, smooth metal bowl mostly just reflects the waves. But the field crowds onto sharp points and thin edges, like the tines of a fork or gold trim on a plate, and it can jump across the air as a spark.

Microwaves are non-ionising. One microwave photon carries about 0.00001 eV of energy, hundreds of thousands of times too little to break a chemical bond. They can heat you, like any heater, but they can't damage DNA the way X-rays or UV can.

A classic magnetron has one setting: full power. So when you choose 50%, the oven switches it on and off: on for 10 seconds, off for 10, over and over. You can often hear the hum change as it clicks. This is called the duty cycle.

Low power is handy for defrosting and gentle cooking. In the off moments, heat has time to spread from hot spots into colder parts, so the edges don't cook while the middle is still frozen.

An inverter microwave has electronics instead of the heavy transformer. It really can run the magnetron at half power, steadily, which cooks more gently and evenly.

Is a microwave efficient? For a mug of water, a kettle wins: its element sits right in the water. But the microwave beats a gas flame, which loses much of its heat around the sides of the pan, and it heats only the food, not a pan or an oven full of air.