How a microwave oven heats food

What microwaves are

The microwaves in a microwave oven are a kind of electromagnetic wave, like light and radio. They belong to the same family as mobile phone and wireless network signals, and most household ovens use a frequency of about 2.45 gigahertz, which corresponds to a wavelength of about 12 centimetres. A component called a magnetron generates the waves and sends them into the cooking chamber, where the metal walls reflect them and keep them inside. The perforated metal mesh in the door does the same job: its holes are far smaller than the wavelength, so microwaves cannot escape while visible light passes through.

Heat from shaking water molecules

A water molecule is polar: one side carries a slight negative charge and the other a slight positive charge. Placed in an electric field, it turns to line up. The electric field of the microwaves reverses direction about 2.45 billion times a second, so water molecules rotate constantly trying to follow it, jostling their neighbours. That motion is heat. Fats and oils warm up too, but food with plenty of water heats best for this reason. A dry dish, by contrast, barely warms on its own because it holds almost no water; a hot dish usually means heat has passed into it from the food.

The cooks-from-the-inside myth

It is often said that microwaves cook food from the inside, but that is not true. Microwaves are absorbed from the surface inwards, and in moist food most of the energy is absorbed within the outer few centimetres. Deeper parts warm up more slowly as heat conducts in from the outside. That is why a thick piece of meat or a large lump of food can be hot outside and cold in the middle. When the inside does feel hotter, it is usually because surface moisture evaporated and cooled the outside, or because the filling is rich in water and fat.

Why heating is uneven

Microwaves reflecting around the chamber overlap and form a fixed pattern of strong and weak spots. This is called a standing wave, and the strong spots are about half a wavelength, roughly 6 centimetres, apart. The turntable keeps the food moving through this pattern to even things out. Ice absorbs microwaves poorly because its water molecules are locked into a crystal. So in frozen food the parts that thaw first can quickly overheat, and the defrost setting cycles the power on and off to give heat time to spread in between.

• Standing waves create hot and cold spots
• The turntable moves food through the pattern
• Ice heats poorly, so defrosting pauses between bursts

Metal and containers

Metal reflects microwaves. Smooth, thick metal simply reflects them, but thin foil, pointed edges such as fork tines, and gold-rimmed dishes concentrate electric charge and can spark. Sparks can damage the oven or start a fire, so the rule is to keep metal out. Use containers marked as microwave-safe. Some plastics can deform when they touch hot food, especially fatty food. If a container has no marking, it is safer to move the food to heat-resistant glass or a ceramic bowl.

Bursting food and superheated water

Food sealed in a skin or membrane can burst as moisture inside turns to steam and pressure builds. Raw or boiled eggs in their shells are the classic case; pricking sausages or potatoes with a fork in a few places makes them less likely to burst. Leave the lid of a sealed container slightly open. When heating plain water, watch out for superheating. Water in a smooth cup can pass its boiling point without forming bubbles, then boil violently the moment you lift it out or put a spoon in. Do not heat it longer than needed, and wait a moment before handling it.

How to heat evenly

Once you know the principle, the techniques follow. Because microwaves enter from the outside, spread food thin and even, and arrange it in a ring with the centre empty to reduce the cold middle. Stir or turn the food halfway through to smooth out the standing-wave pattern. Letting it stand for a minute or two afterwards allows heat from hot spots to spread to cold ones.

• Spread food thin and leave the centre empty
• Stir or turn once halfway through
• Leave the lid slightly open for steam
• Let it stand so heat can spread

What to know about safety

Microwaves are not ionising radiation like X-rays, so they do not make food radioactive or change it in some specially harmful way. When the oven switches off, the microwaves are gone at once. If the door is bent, or the hinges, latches or door seal are damaged, stop using the oven and have the manufacturer's service repair it. Running it empty can damage it, since nothing absorbs the energy. Heated food can vary a lot in temperature from outside to inside, so when serving it to children, stir it and check the temperature yourself.