How can a microwave oven heat food without fire?

KohakuThere is no fire or hot plate inside a microwave oven, yet it warms your rice until it's nice and hot. How does it do that?

The answer in 30 seconds

The star of the story is an invisible relative of radio waves.

A microwave oven sends microwaves, a relative of radio waves, into food and makes the water and other molecules inside it shake hard. As the molecules shake and rub against each other, they create heat, so the food warms up without any fire. The oven makes use of the way microwaves bounce off metal, pass through glass and paper, and are absorbed by food.

A microwave oven for home use — A home microwave oven. The inside is made of metal, which reflects microwaves.Photo: Wrightmt / Public domain / Wikimedia Commons

Microwaves are cousins of radio waves

According to the U.S. Food and Drug Administration (FDA), a microwave oven makes microwaves with a device called a "magnetron." Microwaves are a kind of electromagnetic wave, close to the radio waves used for radio and TV. The U.S. Department of Agriculture (USDA) explains that home microwave ovens use microwaves of about 2450 megahertz, and that the distance from one wave crest to the next is about 10 cm.

Shaking water molecules into heat

Microwaves at this frequency are easily absorbed by water, fat, and sugar. When they are absorbed, the molecules shake back and forth very fast and rub against the molecules around them, which creates heat. According to the FDA, this is why foods with a lot of water, such as vegetables, heat up quickly.

Trapped in a metal box

Microwaves bounce off metal, but they pass through glass, paper, and plastic. The inside of a microwave oven is made of metal, so the microwaves bounce around inside it and are absorbed by the food. Food wrapped in aluminum foil does not heat up because the microwaves cannot reach inside.

Figure: Microwaves inside a microwave oven

The numbers come from the USDA explanation, the same as in the article.

KohakuSo the microwaves keep bouncing around inside a metal box.

Not heating from the inside

People sometimes say that a microwave oven "heats food from the inside." But according to the USDA, microwaves only reach about 2.5–4 cm below the surface. Deeper parts warm up as the heat made near the outside spreads inward. Because food tends to heat unevenly, it is a good idea to stir it partway through or let it sit for a little while before eating.

Want to know more?

Heating food in a microwave oven does not make it radioactive. The energy of the microwaves is simply absorbed by the food and turns into heat.
The air inside the oven does not get hot, so a microwave oven alone has a hard time making the surface of food golden brown or crispy.
When you set the power to "medium," the oven switches the microwaves on and off again and again to lower the average power.

KohakuMicrowaves shake water and other molecules to make heat. They don't heat food from the inside, so stirring partway through is the trick.

Quick quiz: test yourself

How does a microwave oven heat food?

Show the answer

It shakes water molecules in food
It heats the air inside the oven
It heats food from the center first

It shakes water molecules in food Microwaves make the water and other molecules in food shake hard, and as they rub together, heat is made. The air inside does not get hot.

Sources

Microwave OvensU.S. Food and Drug Administration (FDA)
Cooking with Microwave OvensUSDA Food Safety and Inspection Service

This article was written after checking the sources above as of 2026-10-03. Explanations may change as research progresses. Photo credits are shown below each photo.