Heat a slice of pizza and the cheese turns to lava while one corner of the crust stays cold. Heat a bowl of soup and the middle is warm while the edge scalds. The oven is working perfectly. The cold spots are built into how it works: the waves that heat your food are laid out in a pattern, and some places in that pattern get almost nothing.
The part that makes the heat is a tube called a magnetron. It produces radio waves that wobble 2.45 billion times a second, a frequency you can find on a label on the back of most ovens. Radio waves travel at the speed of light, 299,792,458 metres per second, so each wobble is spread over a length of c ÷ f ≈ 12.2 cm. That is the wavelength.
The walls of the oven are metal, so they bounce those waves back. The window has a metal mesh with holes far smaller than 12.2 cm: visible light slips through the holes so you can watch your food, but the long microwaves cannot.
Waves going one way meet the reflected waves coming back. Where a crest meets a crest, they add up. Where a crest meets a trough, they cancel. The result does not travel anywhere. It stands in place: strong at some spots, close to zero at others. Physicists call it a standing wave, and the strong spots and quiet spots sit half a wavelength apart, about 6.1 cm.
Water, fats and sugars soak up energy from the wave where it is strong. Where the wave cancels, there is very little to soak up, so those places barely warm. A real oven is a three-dimensional box and many patterns overlap, so the real layout is messier. The model below is flat and simple, but it keeps the one number that matters: hot spots half a wavelength apart.
Press Start with the turntable off and watch the hot spots appear. Then press Reset, switch the turntable on and run it again. The faint green glow is the wave pattern in the box.
Simplified flat model of a cavity about 30 cm wide, with a 28 cm plate. Real ovens are three-dimensional and less tidy.
The turntable does not change the wave. It moves the food through it, so every bit of the plate spends some time in strong zones and some in quiet ones, and the average evens out. Some ovens add a spinning metal paddle, a stirrer, that scatters the waves around instead.
In the model, you may notice that rings of hotter and cooler food remain even with the turntable on. A bit of food at the very centre never moves, so it only gets whatever the centre of the pattern gives it. That is why stirring partway through and leaving the food to stand for a minute still help: standing lets the heat spread by ordinary conduction into the cooler parts.
Here is a classic kitchen experiment. Stop the plate from spinning, put in a flat bar of chocolate (marshmallows or cheese slices work too) and heat it just until a few spots melt. Never run an oven with nothing in it. Those melted spots are the hot spots of the standing wave, so the distance between the centres of two neighbouring ones is half a wavelength, about 6 cm.
Double it to get the wavelength, about 12 cm, then multiply by the frequency on the label, 2,450 MHz. One published version of the experiment gets 0.12 m × 2,450,000,000 Hz = 294,000,000 m/s. The true value is 299,792,458 m/s, so a ruler and a chocolate bar land within about 2%. Turn the ruler on above, drag A and B to neighbouring hot spots, and the readout does the same sum.
It works because microwaves and light are the same kind of wave, called electromagnetic waves, and they travel at the same speed. Only the wavelength differs.
First, microwaves do not cook food from the inside out. They only reach a few centimetres into food, so thick food is finished by heat conducting inward. Second, 2.45 GHz is not a special frequency that makes water molecules resonate. It sits in a band set aside internationally for industrial, scientific and medical use, and food absorbs microwaves across a wide range of frequencies.
Stir halfway. Let food stand. Spread food out flat rather than piling it in a heap, and put thicker pieces toward the edge of the plate, where they sweep a longer path through the pattern. And if you ever need the speed of light and have only a microwave and a bar of chocolate, you now have enough.