How does a microwave heat food?

The principle

1. A tube makes invisible waves. At the side of the oven sits a small tube with no air inside. In its middle is a thin wire that glows hot, and around the wire is a thick copper ring with a row of small hollows (little chambers) cut into it.

• The hot wire gives off a stream of tiny charged specks called electrons, a bit like steam boiling off hot water. They fly outwards towards the copper ring.
• Two magnets, one above the tube and one below, bend their path, so instead of flying straight out they swirl round in a circle.
• As the electrons rush past the openings of the hollows, the hollows start to “ring” with electricity, much like a bottle hums when you blow across its top. The size of the hollows sets how fast they ring: about 2450 million times a second. That number is written 2450 MHz (“megahertz” just means “million times a second”).
• The ringing sends out invisible waves, a bit like radio waves. A rectangular metal channel guides them from the tube into the cooking box. The waves bounce off the channel’s walls all the way, so they cannot leak out on the way.

You can often see this number for yourself: the rating label on the back of the oven, or inside the door frame, usually says “2450 MHz”, along with how much power the oven gives out (for example 800 W).

2. Metal walls bounce them around. The waves bounce off metal, so the walls keep them bouncing around inside the box. They pass straight through glass, paper and most plastics, which is why those dishes stay fairly cool. The door has a metal net with tiny holes. The holes are far smaller than the waves (which are about 12 cm long), so the waves cannot get out, but light can, so you can still see in.

3. Water molecules twist. Food is full of tiny water molecules. One end of each is slightly positive and the other slightly negative. The waves push them one way, then the other, about 2450 million times a second (the video shows it slowed right down). The molecules twist back and forth and jostle their neighbours, and that jostling is heat.

4. Hot spots, so the plate turns. The bouncing waves add up in some places and cancel each other out in others, leaving hot and cold spots. The turning plate carries the food through them, so it heats more evenly.

5. Outside first, then inwards. Picture a bowl of soup cut in half and seen from the side. The waves only reach about a centimetre or so into the food, from every side they can get at: the surface on top, the sides and the bottom. The middle warms up as heat spreads in from there. That is why it helps to stir, or to let food stand for a minute after heating.

Why does a fork spark, when the metal walls don’t? The walls are smooth, thick and joined up all the way round, with no sharp points. When the waves hit them, the electric charges in the metal can spread out evenly, so the waves are simply bounced back. A fork, or a crumpled piece of foil, is different. Its sharp tips, edges and thin creases make the charges crowd together at the points. If enough crowd together, the air next to the tip can no longer hold them back, and they jump through it as a little spark, like a tiny bolt of lightning. Very thin foil can also get hot, because the waves push a lot of electricity through very little metal.