How Microwave Ovens Work: Science, History & Technology

Introduction to Microwave Ovens

The microwave oven is one of the great inventions of the twentieth century, found in millions of homes across America. Microwave ovens are popular because they cook food incredibly quickly. They are also extremely efficient in their use of electricity because a microwave oven heats only the food and nothing else.

How Microwaves Heat Food

A microwave oven uses microwaves to heat food. Microwaves are radio waves. In the case of microwave ovens, the commonly used radio wave frequency is roughly 2,500 megahertz, or 2.5 gigahertz. Radio waves in this frequency range have an interesting property: water, fats, and sugars absorb them. When absorbed, they are converted directly into atomic motion — in other words, heat. Microwaves in this frequency range have another notable property: most plastics, glass, and ceramics do not absorb them. Metal reflects microwaves, which is why metal pans do not work well in a microwave oven.

In most materials, microwave-power absorption is proportional to the water content of the material. The frequency of commercial microwave ovens (2.45 GHz) was selected so that a standard portion of food would be heated uniformly. Because the heat does not have to be conducted thermally through the food but is generated inside the material itself, microwaving reduces the time needed to bring food to a uniform temperature.

People often hear that microwave ovens cook food "from the inside out." Consider baking a cake in a conventional oven. Normally you would bake at around 350°F, but if you accidentally set the oven to 600°F, the outside of the cake would burn before the inside even gets warm. In a conventional oven, heat migrates by conduction from the outside of the food toward the middle. Dry, hot air on the outside of the food also evaporates moisture, which is why the outside can be crispy and brown while the inside remains moist.

Microwave vs. Conventional Cooking

In microwave cooking, the radio waves penetrate the food and excite water and fat molecules fairly evenly throughout. There is no heat migrating toward the interior by conduction — instead, heat is generated everywhere at once because the molecules are all excited simultaneously. There are limits, of course. Radio waves penetrate unevenly in thick pieces of food and do not always reach the very center. There are also "hot spots" caused by wave interference. Even so, the overall heating process is fundamentally different because you are exciting atoms rather than conducting heat from outside in.

Because the air inside a microwave oven remains at room temperature, there is no way to form a crust. That is why foods like "Hot Pockets" come with a small cardboard and foil sleeve. When placed in the sleeve and microwaved, the sleeve reacts to microwave energy by becoming very hot, allowing the crust to become crispy in a way that resembles conventional oven cooking.

Microwave technology evolved out of the development of radar (Radio Detection And Ranging). Because microwave pulses can be very short, they can be used for distance and time measurement. The simplest form of radar measures the time for an echo to return from a given direction. Microwaves penetrate fog and clouds, travel in straight lines, and produce distinct shadows and reflections.