Appliance Science: The tasty physics of microwave ovens

Ever since humans first dropped a chunk of mammoth meat onto a fire and liked the result, we have been looking for new ways to cook things. One of the more recent developments in this race to taste is the microwave oven, which uses microwave radiation to heat and cook your food.

Because it uses less energy and is much quicker than a gas or electric ovens, the microwave has found a spot in most homes. In fact, it is thought that that by the beginning of the 21st century, over 90 percent of homes in the US had a microwave oven. That's pretty good progress for a device that was invented by accident 55 years ago.

The microwave oven had its origins in radar research before and during World War II. As part of this research, engineers at Raytheon in the US built a large magnetron, a device that generates microwaves. While walking past a magnetron being tested, an eccentric engineer called Percy Spencer noticed that the radiation melted his candy bar. Intrigued, he used this device to cook popcorn and an egg. In the first (but by no means the last) microwave cooking accident, the egg exploded, showering an inquisitive colleague with egg and boiling water. Undeterred, Spencer attached the magnetron to a metal box, and the first microwave oven was born. The device was patented in 1945 by Spencer for the Raytheon Corporation.

Waves & microwaves

The secret of the microwave is in, well, the microwaves. As the name suggests these devices use microwave radiation, with a frequency of about 2.4GHz and a wavelength of about 4 inches (10.16 cm). Created in a device called the magnetron, these electromagnetic waves get water excited. When a microwave hits a water molecule, the molecule absorbs it. This excitement means it moves faster, and the water heats up. And hot water in your food means, as the heat spreads, hot food. This is why microwaves are so good at re-heating food: rather than heating the food from the outside like a normal oven would, they heat the water within the food, so it heats up quicker and more evenly. The microwaves also penetrate into the food, effectively heating it from within.

Of course, you don't want things outside the oven getting heated up, which is why they are made of metal. This metal cage traps the microwaves inside the oven: they bounce around from wall to wall until they hit something that can absorb them.

"But wait!" I hear you cry. "What about the door? That is made of glass that I can see through! Does that mean every time I look through this my eyeballs are being boiled?"

Fortunately, the answer is no. If you look closely, you'll see a grid of wires inside the clear glass door. These are connected to the metal walls of the oven, and block the microwaves from getting out. It is one of the odd quirks of radiation that if a hole is smaller than the wavelength of the radiation, most of it won't pass through the hole. So, to the microwaves, this grid of wires looks like a solid wall, and most of it keeps bouncing around inside the oven until it hits an excitable water molecule. It's the same reason why broadcast TV antennas made of grids of wire work: the grid looks like a solid surface to the radiation.

This is also the reason why microwaves are built so they won't work when the door is open, and why messing with this is a VERY BAD IDEA. Likewise, with all the tricks you see on YouTube about putting CDs or metal objects in microwaves. Cooking metal objects in a microwave is ANOTHER VERY BAD IDEA because the microwave can induce an electric current in metal. This current has got to go somewhere, and the easiest path between bits of metal is sometimes through the air (such as between the tines of a fork). This creates a plasma of ionized gas which can start a fire.

The reason you get the oddly beautiful effect when you microwave a CD is that the radiation induces a current in the metal, and the small holes that the data is stored on are just wide enough to create a strong electric current which arcs across the gap, melting the metal and the plastic coating. As the metal film that the data is stored on in the CD melts, the arc moves around the data track of the CD. But just to say it again, DON'T TRY THIS AT HOME, JUST WATCH IT ON YOUTUBE.

The microwaves themselves are created inside the magnetron. You can usually locate this in your microwave by looking for a plastic panel inside the cooking cavity. This panel covers the magnetron output, where the microwaves come from. The magnetron generates microwave radiation by bouncing electrons around inside a vacuum filled cavity that is exposed to a strong magnetic field. This magnetic field forces these electrons to circle around inside the cavity, absorbing energy. Eventually, this energy is released as a microwave. This microwave radiation is then gathered and directed into the cooking space of the microwave by a device called a wave guide.

Some ovens also feature a rotating metal fan (called the stirrer) which spreads the microwave beam to make it more random. This process of generating microwaves requires a very high voltage, usually in excess of a thousand volts (1 kilovolt). And that's another reason why you shouldn't mess with microwave ovens, as that voltage could easily kill you.* That's not why it is called a kilovolt, but it is a handy reminder of why you shouldn't take microwave ovens apart.

Once the microwaves are inside the cooking cavity, they bounce around until they hit something that can absorb them, such as a water molecule. Like waves in a bathtub, this reflection produces peaks (with lots of microwaves) and troughs with few microwaves, which scientists call a standing wave. This is why your microwave has a rotating dish in it, as this rotation makes sure that none of the food is sitting in a trough on this pattern, not being heated.

This is also why you need to be careful about both the food and the dishes you use in a microwave. If the food doesn't have any water in it, it won't absorb microwaves and it won't heat up. Many older plates and other dishes may contain some water or other materials that absorb microwaves, which means you'll end up heating the dish and not the food. The same is true of cracked or damaged dishes, where water can get into the crack and expand rapidly as it is heated, breaking the dish. In both cases, the results can be explosive, so only use dishes that are marked as microwave safe.

Wi-Fi is microwave too

One thing you might have noticed is that the 2.4GHz frequency of the microwaves used in microwave ovens sounds familiar. You'd be right: this is the same frequency as the 802.11g or n wireless routers you use in your home or office. Your Wi-Fi devices use the same frequency, but at much lower power: while a typical microwave oven can generate several hundred watts of microwave radiation, your Wi-Fi devices are emitting only a few milliwatts of radiation. That's not enough to cook sushi.

It does explain, though, why your Wi-Fi devices sometimes stop working when someone makes popcorn; although most of the microwave radiation is contained by the oven, a small amount escapes and can overwhelm the Wi-Fi signal. Newer Wi-Fi devices that use the 802.11ac standards don't have this problem, as they send and receive data at a higher frequency of 5GHz.

The future of microwaves

Microwave heating remains the most efficient way to reheat food, and there is nothing on the horizon at present that looks likely to challenge that. The basic mechanism of the microwave hasn't changed much in the past 40 years, since the development of the cavity magnetron in the 1960s. What has happened is that microwave ovens have got cheaper and cheaper, thanks to the increasing efficiency of manufacturing.

The cheapest microwave oven I could find on sale at the time of writing cost under $50. So, microwave ovens are likely to remain a mainstay of the modern kitchen for many years to come.* Pedants among you may point out that a high voltage won't kill you, but a high current will. As my physics teacher used to say, "It's the volts that jolts, and the mills that kills", because a high voltage won't kill you, but a small amperage (in the milliamp range) across the heart for a couple of seconds is enough to stop your heart beating and kill you. Either way, don't mess with big voltages.