A conventional oven heats food very slowly from the outside in, but a microwave oven uses tiny, high-powered radio waves to cook food more evenly (loosely speaking, we sometimes say it cooks from the "inside out"—although that isn't quite correct). This is why a microwave can cook a joint of meat roughly six times faster than a conventional oven. Microwave ovens also save energy, because you can cook immediately without waiting for the oven to heat up to a high temperature first. Let's take a closer look at how they work!
Microwave ovens are so quick and efficient because they channel heat energy directly to the molecules (tiny particles) inside food. Microwaves heat food like the sun heats your face—by radiation.
A microwave is much like the electromagnetic waves that zap through the air from TV and radio transmitters. It's an invisible up-and-down pattern of electricity and magnetism that races through the air at the speed of light (300,000 km or 186,000 miles per second). While radio waves can be very long indeed (some measure tens of kilometers or miles between one wave crest and the next), they can also be tiny: microwaves are effectively the shortest radio waves—and the microwaves that cook food in your oven are just 12 cm (roughly 5 inches) long. (You can read more about electromagnetic waves in our article on the electromagnetic spectrum.)
Despite their small size, microwaves carry a huge amount of energy. One drawback of microwaves is that they can damage living cells and tissue. This is why microwaves can be harmful to people—and why microwave ovens are surrounded by strong metal boxes that do not allow the waves to escape. In normal operation, microwave ovens are perfectly safe. Even so, microwaves can be very dangerous, so never fool around with a microwave oven.
How does a microwave turn electricity into heat? Like this!
- Inside the strong metal box, there is a microwave generator called a magnetron. When you start cooking, the magnetron takes electricityfrom the power outlet and converts it into high-powered, 12cm (4.7 inch) radio waves.
- The magnetron blasts these waves into the food compartment through a channel called a wave guide.
- The food sits on a turntable, spinning slowly round so the microwaves cook it evenly.
- The microwaves bounce back and forth off the reflective metal walls of the food compartment, just like light bounces off a mirror. When the microwaves reach the food itself, they don't simply bounce off. Just as radio waves can pass straight through the walls of your house, so microwaves penetrate inside the food. As they travel through it, they make the molecules inside it vibrate more quickly.
- Vibrating molecules have heat so, the faster the molecules vibrate, the hotter the food becomes. Thus the microwaves pass their energy onto the molecules in the food, rapidly heating it up.