Your turn
Each lab is one step from the video. Tap, slide and drag, and see what happens.
Each lab isolates one idea. The models are simplified to show it.
Tap the buttons and slide the sliders.
Start the oven. Which thermometer climbs, the soup or the plate?
Run the oven. Why does the food heat while the dish barely warms?
Press start. What gets hot, the soup or the plate?
Put glass, paper, soup or a fork in the wave's path. Which one soaks it up?
Swap the material. Which transmits, which absorbs, which reflects?
Tap the glass, the paper, the soup and the fork. What happens to the wave?
Flip the field or switch the microwaves on. What happens to the temperature?
Drive the dipoles with the field. How does rotation become heat?
Switch the waves on. Watch the tiny bits wiggle and warm up!
Make the holes in the door screen bigger. When do the waves escape?
Scale the mesh openings up toward the 12 cm wavelength. When does the shielding fail?
Switch the turntable off and cook the chocolate. How far apart are the melted spots?
Turntable off: measure the antinode spacing and work out the wavelength.
Turn the spinning plate off and cook. Where does the chocolate melt?
Drag the person away from the oven. How much of the tiny leak limit can reach you?
Move away from the oven. How fast does the maximum possible exposure drop?
A microwave oven makes invisible waves. They are a kind of radio wave. The waves bounce around inside the metal box.
Long ago, a man named Percy Spencer stood next to a radar machine. The chocolate in his pocket melted! Then he held up popcorn, and it popped. That is how the microwave oven began.
Food has water in it. The waves push and pull on the tiny bits of water. They wiggle one way, then the other. They bump into each other, and that makes the food warm.
A plate has almost no water in it, so most of the waves go right through it. It warms mainly from the hot food sitting on it, and that can still make it hot, so let a grown-up lift it.
The waves make hot spots and cool spots. The plate inside turns round and round, so the food warms more evenly. Some parts can still be cooler, so stir it.
A microwave oven is a metal box filled with microwaves, a kind of radio wave. A part called a magnetron makes them at a frequency of about 2.45 gigahertz (2,450 million waves a second).16 The metal walls bounce the waves around until the food soaks them up.1 Inside the food, the waves' electric field flips back and forth and twists the water molecules one way, then the other. As those molecules jostle their neighbours, the food gets hot.23
That's the whole trick: the oven doesn't heat the air or the box. It heats the water, fat and sugar inside the food.6
Microwaves are electromagnetic waves, the same family as radio, visible light and X-rays. They sit at the high-frequency end of the radio range.310 Like all electromagnetic waves, they travel at the speed of light.7 The ones in your kitchen are about 12 centimeters long from crest to crest, roughly the length of a pencil stub.34
The same kind of waves do lots of other jobs. They carry phone calls, WiFi and Bluetooth, bounce off planes and storms in radar, and even let satellites see through clouds.23
| Wave | Typical wavelength | Can it knock electrons off atoms? |
|---|---|---|
| Radio / microwaves | 1 mm to many meters (ovens: ~12 cm) | No (non-ionizing) |
| Visible light | ~0.0005 mm | No (non-ionizing) |
| X-rays | far shorter than light | Yes (ionizing) |
Wavelength ranges from Britannica and HyperPhysics; ionizing vs non-ionizing from the FDA.1410
The magnetron is a vacuum tube that turns electricity into microwaves.16 A hot wire in the middle gives off electrons. A strong magnet bends their path so they whirl around the tube past a ring of small metal cavities. The swirling electrons make electric charge slosh back and forth in those cavities, which then "ring" at their natural frequency and send out microwaves, a bit like blowing across the top of a bottle makes it hum.5
Magnetrons were built for radar during World War II. That's how the microwave oven was discovered: soon after the war, Raytheon engineer Percy Spencer noticed that a chocolate bar in his pocket had melted while he stood near a working magnetron. He then held popcorn kernels near it, and they popped.8 Spencer didn't invent the magnetron, but he'd worked out how to mass-produce it.8
A microwave oven relies on three properties of microwaves. They're reflected by metal, they pass through glass, paper and plastic, and they're absorbed by food.1 Food soaks them up because of the water (and fats and sugars) in it.6
A water molecule is lopsided. Its electrons huddle closer to the oxygen atom than to the two hydrogens, so the oxygen end is slightly negative and the hydrogen end slightly positive. Scientists call this a polar molecule, or a dipole.2 A microwave's electric field pushes on those charged ends. The field flips direction billions of times a second, so each molecule is twisted first one way and then the other. Because every molecule is tangled up with its neighbours, all that twisting and tugging turns into random jostling, and that's what heat is.24
This is also why a plate can come out cooler than the food on it. A dry ceramic plate has no water for the waves to twist, so it stays fairly cool, and any warmth it has mostly comes from the hot food sitting on it.12
water molecule (+ and − ends) + flipping electric field → twisting, jostling molecules = heat
No, and this myth is widespread. Ovens don't hit a sharp resonance of water. Water absorbs microwaves over a broad range of frequencies, and its strongest absorption is at a much higher frequency than 2.45 GHz.4 That's actually useful: at 2.45 GHz the waves aren't all swallowed at the surface, so in water at room temperature about 63% of their power is absorbed in the first 1.4 cm (the exact depth depends on the food and its temperature).4
No. The heat is made inside the food, but mainly in its outer layer. In thick foods, the outside is heated by the microwaves and the middle is cooked mostly as that heat spreads inward, just like in a regular oven.1 Moist foods heat faster than dry ones, and the outer layers soak up most of the waves, which is one reason big portions can stay cold in the middle.6 Microwaves also can't brown or crisp food.6 That's why the instructions often say to stir, or to let the food stand for a minute.
Look closely at the door and you'll see a metal screen full of tiny holes. The holes are about 1 millimeter across, but the microwaves are about 120 millimeters long, so the waves are far too big to get through and bounce back inside. Visible light waves are thousands of times smaller than the holes, so light passes straight out and you can watch your food.4
The door has a second line of defence. By law, every oven sold in the US needs two independent safety interlocks that stop the microwaves the moment the latch is released or the door opens, plus a monitor that shuts the oven down if an interlock fails.1
Waves bouncing back and forth between the metal walls overlap and form a standing wave, a pattern that stays in place.2 Where the waves add up you get hot spots, and between them are spots that barely heat. The hot spots are half a wavelength apart, about 6 centimeters in a 2.45 GHz oven.29 A turntable carries the food through the hot and cool spots so it heats more evenly. Some ovens use a spinning metal fan called a stirrer that scatters the waves instead.2
With an adult's help, you can see the hot spots for yourself.
0.122 m × 2,450,000,000 per second ≈ 300,000,000 m/s = the speed of light
The speed of light is 3.00 × 108 meters per second,7 so a kitchen and a chocolate bar get you surprisingly close. Your measurement won't be perfect, because the patches are fuzzy and the pattern inside a real oven is three-dimensional.
The FDA has regulated microwave ovens since 1971 and considers ovens that meet its standard, used according to the instructions, safe.1 The World Health Organization says the same: shielding keeps leakage to almost undetectable levels, leakage falls very rapidly with distance, and an oven that meets the manufacturing standards is not a hazard.11 Here's why:
Very large doses of microwaves can burn, just as they heat food, and the eyes are especially sensitive. But that takes a large exposure, such as from a badly damaged oven.1 The FDA's advice: don't use an oven whose door is bent or won't close firmly, and stop using one that keeps running with the door open.1 In real life, most microwave injuries are ordinary burns from hot food, containers and liquids.1
No more than other cooking. According to the FDA, microwaving is quick and uses little water, so food may even keep more of its vitamins and minerals than when it's boiled.1 It can also use less energy, because it heats only the food, not the whole oven.1
Percy Spencer and his employer Raytheon filed a patent on October 8, 1945. The first commercial microwave oven, the Radarange, went on sale in 1947. It cost about $3,000, weighed around 750 pounds and stood 6 feet tall. It took about 20 more years before a microwave small enough for home kitchens was ready.8
A part called a magnetron makes microwaves, a kind of radio wave. They bounce around the metal box and are soaked up by the water, fat and sugar in food. Water molecules have a positive and a negative end, so the microwaves' flipping electric field twists them back and forth billions of times a second. As they jostle their neighbours, the food heats up.
Both, in a way. The oven sends out microwave radiation, which is a kind of radio wave, not the nuclear kind. The food absorbs that energy and it turns into heat inside the food. The oven itself doesn't heat the air the way a regular oven does.
Yes, when they're in good shape and used as the instructions say. The FDA limits how much can leak out to 5 milliwatts per square centimeter at about 2 inches from the oven, far below levels known to harm people. Two separate safety switches turn the microwaves off the moment the door opens. Most microwave injuries are ordinary burns from hot food or liquids.
No. Microwaves are non-ionizing: they don't have enough energy to knock electrons out of atoms the way X-rays can. The energy simply becomes heat, and the food isn't radioactive or contaminated afterwards.
Metal reflects microwaves. Food in a metal pan barely heats, it cooks unevenly, and it can damage the oven. Thin metal like aluminum foil can also heat up very fast and even start a fire.
Waves bouncing between the metal walls form a standing wave, with hot spots about 6 cm apart and cooler spots in between. The turntable moves the food through those hot and cool spots so it heats more evenly. Some ovens use a spinning metal fan called a stirrer instead.
No more than other cooking methods, according to the FDA. Because microwaving is quick and uses little water, food can even keep more of its vitamins and minerals.