The microwave oven is one of the most common appliances in the world. It reheats leftovers in a minute, softens butter, steams vegetables and makes popcorn. Yet many people are unsure how it actually works. Does it cook food from the inside out? Is it safe? Why do some parts of a plate stay cold while others become scorching hot, and why does metal spark? The answers come from the physics of electromagnetic waves and the behavior of water molecules. This explainer takes you inside the microwave oven.
A happy accident
The microwave oven was invented almost by accident. In 1945, an American engineer named Percy Spencer was working on radar equipment that used a device called a magnetron to produce microwaves. He noticed that a chocolate bar in his pocket had melted. Curious, he tried placing popcorn kernels near the magnetron, and they popped. The first commercial microwave oven followed in 1947. It was nearly 6 feet (1.8 meters) tall and weighed about 750 pounds (340 kilograms). Countertop models became popular in homes in the 1970s and 1980s.
What are microwaves?
Microwaves are a type of electromagnetic radiation, the same family as radio waves, visible light, infrared, ultraviolet and X-rays. They differ only in their wavelength and energy.
Microwaves have wavelengths from about 1 millimeter to 1 meter. Most household microwave ovens use a frequency of about 2.45 gigahertz, meaning the waves oscillate 2.45 billion times per second, with a wavelength of about 4.8 inches (12.2 centimeters). Wi-Fi networks also commonly use frequencies around 2.4 gigahertz.
Microwaves are non-ionizing radiation. Unlike X-rays or ultraviolet light, they do not have enough energy to knock electrons out of atoms or damage DNA directly. Their main effect is to make molecules move and heat up.
Inside the oven
A microwave oven has a few key parts:
- The magnetron: A vacuum tube that converts electricity into microwaves. Inside, electrons spiral in a magnetic field past cavities in a copper block, causing the cavities to resonate and produce microwaves.
- The waveguide: A metal channel that directs microwaves from the magnetron into the cooking chamber.
- The cooking chamber: A metal box that reflects microwaves back and forth so they pass through the food many times.
- The turntable or stirrer: Rotates the food or scatters the microwaves to distribute heating more evenly.
- The door: Contains a metal mesh screen that blocks microwaves while letting you see inside.
How microwaves heat food
Food contains water, fats and sugars. Water molecules are polar: one end has a slight positive charge and the other a slight negative charge, like tiny magnets.
Microwaves are made of oscillating electric and magnetic fields. As the electric field rapidly changes direction billions of times per second, polar water molecules try to line up with it, rotating back and forth. As they spin and jostle, they collide with neighboring molecules, transferring energy. This increased molecular motion is what we experience as heat. This process is called dielectric heating.
Fats and sugars also absorb microwave energy, though differently from water. Dry materials, such as paper, glass, ceramic and many plastics, absorb little microwave energy, which is why they stay relatively cool unless heated by the hot food they contain.
Does a microwave cook from the inside out?
This is a common myth. Microwaves penetrate food only to a depth of about 0.4 to 1.5 inches (1 to 4 centimeters), depending on the food. The outer layers heat directly, and the heat then spreads to the center by conduction, just as in conventional cooking. In thick foods, the center can remain cold while the outside becomes hot.
Sometimes it seems as if a dish is hotter inside, for example a jam-filled pastry. That is because the filling contains more water and sugar, absorbs more energy and retains heat longer than the drier outer crust.
Why food heats unevenly
Microwaves bounce around inside the metal chamber, and as they reflect, they create a pattern of standing waves, with areas where waves reinforce each other, producing hot spots, and areas where they cancel, producing cold spots. These spots are a few centimeters apart, related to the wavelength of the microwaves.
You can see this with a simple experiment: remove the turntable, place a plate covered with a thin layer of chocolate or cheese in the oven and heat it briefly. The chocolate will melt in spots spaced about 2.4 inches (6 centimeters) apart, half the wavelength. Some people even use this to estimate the speed of light.
Turntables and metal stirrers help even out the heating by moving the food through hot and cold spots or scattering the waves. Different foods also absorb microwaves differently: ice absorbs microwaves much less than liquid water, which is why frozen food can thaw unevenly, with some areas starting to cook while others remain frozen.
Why metal sparks
Metal reflects microwaves. That is why the oven's walls are metal: they keep the microwaves inside. A smooth, thick metal container, such as a pan, simply reflects microwaves and blocks them from heating the food inside.
But thin or pointed metal objects, such as forks, aluminum foil with crumpled edges or the gold trim on decorative plates, can cause problems. The microwaves induce electric currents in the metal, and electric charge concentrates at sharp points and edges. When the electric field becomes strong enough, it ionizes the air, creating sparks or arcs. This can damage the oven or start a fire.
Some microwave ovens come with metal racks designed for safe use, and small, smooth pieces of foil can sometimes be used to shield parts of food, if the manufacturer's instructions allow it.
Is microwave cooking safe?
Microwave ovens are designed with multiple safety features. The metal walls and door screen prevent microwaves from escaping. The holes in the door screen are much smaller than the wavelength of the microwaves, so the waves cannot pass through, while visible light, with much shorter wavelengths, can. Safety interlocks turn off the magnetron as soon as the door opens.
Regulatory agencies limit the amount of microwave radiation that may leak from ovens to a very small level, far below what would harm people. Microwaves do not make food radioactive; once the oven turns off, no microwaves remain in the food.
The main safety concerns are practical:
- Burns from hot food, steam and containers.
- Superheated water: Water heated in a clean, smooth container can sometimes exceed its boiling point without bubbling, then boil violently when disturbed. Place a wooden stirrer in the cup or heat water in shorter intervals.
- Uneven heating: Cold spots can allow bacteria to survive, especially when reheating leftovers or cooking meat. Stir food, let it stand and check that it reaches a safe temperature, such as 165 °F (74 °C) for leftovers.
- Containers: Use microwave-safe containers. Some plastics can warp or release chemicals when heated. Glass and ceramic are good choices.
- Damaged ovens: Do not use an oven with a damaged door, hinges or seals.
Does microwaving destroy nutrients?
All cooking methods affect nutrients. Heat breaks down some vitamins, and water can wash out water-soluble vitamins like vitamin C and some B vitamins. Because microwave cooking is fast and often uses little water, it can preserve nutrients as well as or better than boiling. Steaming vegetables in a microwave with a small amount of water is an efficient way to cook them.
Tips for better microwave cooking
- Cover food with a microwave-safe lid or vented cover to trap steam and heat more evenly.
- Arrange food in a ring, with thicker parts toward the outside, leaving the center empty.
- Stir and rotate food partway through heating.
- Use standing time: Let food rest for a minute or two after heating so heat can spread.
- Reduce power for defrosting and for dense foods, so heat has time to conduct inward.
- Cut food into similar sizes so it heats evenly.
Questions readers ask
Do microwaves make food radioactive? No. Microwaves heat food by making molecules move. They do not make food radioactive, and no radiation remains after the oven turns off.
Why does my food heat unevenly? Microwaves form standing wave patterns with hot and cold spots, and they penetrate only a few centimeters into food. Stirring, covering and standing time help.
Why does metal spark in a microwave? Thin or pointed metal concentrates electric charges, which can ionize the air and create sparks.
Can microwaves leak from the oven? Ovens are designed to keep microwaves inside. Leakage from properly working ovens is extremely low and regulated. Do not use an oven with a damaged door.
Do microwaves cook food from the inside out? No. They heat the outer few centimeters directly, and heat spreads inward by conduction.
In short
A microwave oven uses a magnetron to produce microwaves at about 2.45 gigahertz, which bounce around a metal chamber and make water and other polar molecules in food rotate rapidly, generating heat. Microwaves penetrate only a few centimeters, so heat spreads inward by conduction, and standing waves create hot and cold spots that turntables and stirring help even out. Thin or pointed metal can spark, while the metal walls and door screen keep the microwaves safely inside. Used correctly, the microwave is a safe, fast and efficient way to heat food, and a small daily lesson in physics.