How Does a Microwave Oven Heat Food?

Inside every microwave oven is a magnetron, a vacuum tube invented for radar in 1940, which turns electricity into electromagnetic waves at about 2.45 gigahertz. That is a radio frequency, in the same general band as wifi and Bluetooth, with a wavelength of about 12 centimetres. The waves are guided into the cooking chamber, a metal box that reflects them, so they bounce around inside until something absorbs them.

The choice of frequency is a compromise. Water absorbs these waves strongly enough to heat but not so strongly that all the energy is soaked up in the first millimetre, which is what would happen at higher frequencies. It is also a band set aside internationally for industrial and domestic equipment, which is why the oven and the router use it and the radio station does not.

A water molecule is electrically lopsided: its oxygen end carries a slight negative charge and its hydrogen end a slight positive one. In an electric field the molecule tries to line up with the field, and a microwave is an electric field that reverses direction 2.45 billion times a second. The water molecules twist back and forth to follow it, and as they twist they jostle their neighbours, and that jostling is heat.

This is called dielectric heating, and it works on any molecule with an uneven charge distribution, which includes fats and sugars to a lesser degree. It does not work on the glass, ceramic or plastic of a plate, whose molecules are not free to rotate, which is why the food is hot and the plate is warm only where the food touched it. The common claim that microwaves heat food from the inside out is not quite right: the waves penetrate a centimetre or two and heat that layer directly, and the middle of a thick piece of food still warms by conduction from there.

The waves bouncing around the metal chamber interfere with each other, adding up in some places and cancelling in others, and the pattern of peaks and dead zones is fixed by the box's dimensions. Food sitting at a dead zone stays cold while food a few centimetres away scorches. The turntable exists to carry the food through the pattern, and ovens without a turntable use a rotating metal stirrer to move the pattern over the food instead.

You can map the pattern yourself by laying a slab of chocolate or a sheet of marshmallows in a stopped oven for a short burst: the melted spots are the peaks, and their spacing, about six centimetres, is half the wavelength. Dividing the wavelength by the time for one wave cycle gives the speed of light, and this is a standard school experiment for exactly that reason.

Metal reflects microwaves rather than absorbing them, which is why the oven's walls are metal and why a smooth metal bowl does little harm. Trouble comes from points and edges. The electric field concentrates at a fork's tines or the crinkled edge of foil until it is strong enough to tear electrons from the air, and the result is a spark. A thin metal decoration on a plate can heat and crack the glaze the same way.

Eggs and whole potatoes explode for a different reason. Steam forms inside faster than it can escape through the skin or shell, pressure builds, and the food bursts, sometimes after it has been taken out. A cup of water heated in a very clean, smooth mug can go above its boiling point without boiling, because there are no rough spots for bubbles to form on, and then erupt violently when a spoon or a teabag is dropped in. Stirring before heating and letting the cup stand for a moment afterwards prevents it.

Microwaves are not ionising radiation. Their photons carry far too little energy to break chemical bonds or alter DNA, unlike X-rays or ultraviolet light, and they do not make food radioactive any more than a light bulb makes a table luminous. What they can do is heat tissue, which is why the door has a metal mesh with holes far smaller than the 12-centimetre wavelength, so light passes through and microwaves cannot, and why the oven switches off when the door opens.

Nutritionally, microwaving is one of the gentler cooking methods. Vitamins are destroyed by heat and by time in water, and a microwave uses little of either, so steamed-in-the-bag vegetables generally keep more of their vitamin C than boiled ones. The process, step by step:

• •The magnetron produces 2.45 gigahertz radio waves
• •The metal chamber reflects them until they are absorbed
• •Water molecules twist to follow the reversing field
• •The twisting molecules heat the outer layer of the food
• •Conduction carries the heat inward, helped by standing time

Defrosting is the job microwave ovens do worst, and the reason is that ice absorbs microwaves far less than liquid water. In ice the water molecules are locked into a rigid crystal and cannot twist freely to follow the field, so a frozen block heats slowly at first. The moment any part of it melts, that liquid patch absorbs energy much faster than the ice around it, gets hotter, and melts its surroundings faster still. Left on full power, you get the familiar result: cooked edges around a core that is still frozen solid.

The defrost setting deals with this by switching the magnetron on and off in cycles rather than running it at lower strength, since a basic magnetron is either on or off. During the off periods, heat from the thawed patches spreads by conduction into the colder parts, evening things out before the next burst. Some newer ovens use inverter power supplies that can deliver a genuinely lower continuous power. Either way, the advice to turn or separate food partway through and to let it stand afterwards follows from the same physics: you are giving conduction time to catch up.

The microwave oven came out of wartime radar work. According to the story usually told, in 1945 Percy Spencer, an engineer at the American company Raytheon who worked on magnetrons, noticed that a chocolate bar in his pocket had melted while he stood near a working one. He tried pointing a magnetron at popcorn kernels, which popped, and at an egg, which exploded. Raytheon patented the idea of cooking with microwaves, and within two years it was selling the first commercial microwave oven, which it named the Radarange.

That first machine was nothing like the modern box. It stood roughly as tall as a person, weighed several hundred kilograms, needed water piped in to cool its magnetron and cost thousands of dollars, so it was sold to restaurants, railway companies and ships rather than to households. Shrinking it took two decades of work on smaller, cheaper, air-cooled magnetrons. In 1967 Amana, a company Raytheon had bought, launched a countertop model for home kitchens priced at just under 500 dollars, and over the following decades falling prices made the microwave one of the most common appliances in the world.

Most microwave ovens work at a frequency of about 2.45 gigahertz, in a band of radio frequencies set aside internationally for industrial, scientific and medical uses. Ordinary Wi-Fi and Bluetooth also use the band around 2.4 gigahertz, because it is available without a licence. The oven's metal box and mesh door keep almost all of its energy inside, but a very small amount leaks out.

That leakage is far below any level that could harm people, but it is enough to interfere with nearby wireless signals using the same frequencies. Some people notice video calls stuttering or Wi-Fi slowing when the microwave is running, especially if the router sits close to the kitchen. Moving the router, or using the 5 gigahertz Wi-Fi band, usually solves the problem.

A microwave oven heats food by making its water molecules rotate billions of times a second in a rapidly reversing electric field. Plates stay cool because their molecules cannot rotate, cold spots come from the wave pattern in the box, and sparks come from metal points concentrating the field, while the waves themselves are far too weak to change the food chemically.