As its name suggests, a microwave oven is an oven that heats food with microwaves (a specific wavelength of radio waves). This has led to a certain amount of concern among people who don't know how stuff works.
How stuff really works
Microwave ovens are a mystery for most people. After all, you just put your food inside a box, press a button, wait some time, and then voilà, your food is neatly cooked. Microwaves (electromagnetic waves, not ovens) were produced way back in the 1800s in some of the earliest radio experiments. Heinrich Hertz was the first person to demonstrate their existence. They have been used ever since and have found applications in many branches of science and technology. The ability of microwaves to heat up food was discovered by an accident in 1945 by Percy Spencer, an American physicist, and inventor. He was experimenting with a magnetron, a device that generates microwaves, and he also happened to have a candy bar in his pocket which melted during the experimentation. He soon began investigating this curious fact. He became the first person to microwave popcorn and he did it in the name of science! The first commercial microwave, although very extensive in size, was called 'Radarange' and marketed in 1947.
The basic idea behind Radarange and all modern microwave ovens is this: magnetrons generate microwave radiation when powered by electricity, this radiation is trapped in a metal housing (similar to a Faraday cage), so that it amplifies the heating effect and does not escape from the oven. Since microwaves are electromagnetic waves they affect polar molecules and ions. (A polar molecule is a molecule in which one end of the molecule is slightly positive, while the other end is slightly negative; in other words, they have unevenly distributed electrical charge). Examples of polar molecules are water (H2O), ammonia (NH3), or sulfur dioxide (SO2) and many others. These molecules are constantly being aligned with the waves, once the oven is running. This happens some 2.45 billion times per second, as this is around the frequency of an average microwave. As a result of this, molecules rotate, rub and bounce against each other causing friction and heat.
The fact that only polar molecules and ions respond by aligning themselves to the microwaves in a microwave oven means that things made from nonpolar material will not heat up as a result of microwave absorption. They still can get heated up when they are in contact with other things that are hot or heating up, but microwaves themselves are unable to cook them. For example, porcelain is usually made of kaolin, which has a net zero electric charge and low ion exchange capacity, which in turn means it shouldn't heat up much in a microwave. Cellulose also doesn't have a highly polar structure, so putting dry paper in microwaves usually does not result in combustion. But just in case, do not try this, please.
Oven plate and the speed of light
Modern microwave ovens also have a plate in the center that rotates while the oven is running. This is intentional, and it's because the wavelength of the microwaves used in microwave ovens is about 12 cm (4.7"), so in order to cook food properly and avoid those hot and cold spots you usually get when you place food in the very center of the plate, you should place the things you want to cook on the edge of the plate, then all the food will come across the microwaves. This fact can be used to measure the speed of light! Just turn the plate upside down in the oven so it doesn't rotate and put a bar of chocolate or spread some cheese in the microwave. This way you will fix the position of hot and cold spots on your delicious dish. Run the microwave for a short time, and then measure the distance between the hot spots (places where the chocolate or cheese has melted significantly). This distance will be half the wavelength of the microwaves (for microwaves at 2.45 GHz, you should measure about 6 cm or 2.3 inches). Now use the formula for the speed of a wave (v = λf), where λ is the wavelength (twice what you measured) and f is the frequency, which is written somewhere on the microwave. So 0.12 m × 2.45 GHz = 294,000,000 m/s. Which is pretty close to the speed of light's actual value.
Microwave ovens aren't natural!
Neither are using computers, living in buildings, or wearing clothes. If you want to live in a cave naked, go ahead.
The Nazis discovered microwaves!
Radio waves were first discovered in experiments by German physicist Heinrich Hertz, based on James Clerk Maxwell's electromagnetic theory of light. But Hertz died decades before the Nazis came to power, and worse yet, after they did, tributes to Hertz for his scientific discovery were purged from public view because of his Jewish heritage.
Claims about the Nazis inventing microwave ovens are contradictory and without evidence. Some claims purport that the Wehrmacht used large microwave ovens called "Radiomissor" during Operation Barbarossa, the German invasion of the Soviet Union in 1941. This, of course, conveniently ignores the fact that Russia's electricity grid at that time was very sparse, and the Wehrmacht, due to its rapid advances, already faced great difficulty supplying tanks with fuel, so running electricity generators for the sake of powering microwave ovens seems rather impractical when horse-drawn field kitchens were widely available and the acquisition of firewood trivial.
In contradiction, other claims state the microwave oven was researched by Nazi scientists for the above use but canceled because it was allegedly found that microwave oven food caused cancer (see points below). This again ignores the historical fact that the far more useful military application of microwaves lies in radar, which was still a new technology at the outset of World War II, and it, therefore, would have been unreasonable to direct such research to the purpose of food preparation. It was, in fact, Percy Spencer, a leading researcher of American radar technology, who would invent the microwave oven shortly after the war, and its development by the Raytheon company is well documented.
Last but not least, if there were a moral imperative that we shouldn't use things the Nazis invented, we would also have to stop driving cars on highways, flying around in jet planes, depending on satellites shot into space with rockets, listening to music produced with multitrack recording, etc.
Did James Lovelock really invent the microwave oven?
Dr. Lovelock does claim this on his website and certainly did it before it was commercial and widely available. However, the microwave oven was patented in 1945; Dr. Lovelock's earliest claim was in a 1955 journal article on the reanimation of cryonic preserved rats using microwave diathermy.
Why did the Soviet Union ban microwave ovens in 1976?
This is a widely purported myth, but there is no evidence it ever happened. The ban was supposedly based on Soviet research linking cancer growth to food cooked in microwave ovens, but no trace of any such research exists either.
Heat can destroy a few vitamins and other nutrients in food. Boiling in water can also remove nutrients because the nutrients migrate out of the food and into the water, which is thrown away. Since microwave cooking usually involves shorter cooking times and less boiling than conventional cooking methods, it destroys fewer nutrients than conventional cooking. This doesn't mean either cooking method is "bad for you," though — you probably get enough essential nutrients from other sources anyway, without having to drag raw foodism into it.
No more so than heating food in a conventional oven does. Yes, some trace compounds created in baking are carcinogenic when ingested in sufficient quantities. But the amounts of carcinogens created by heating food — in a microwave or otherwise — are well, well below the threshold where they can have any impact on your chance of getting cancer. Unless you heat it so much that it turns into a lump of charcoal, in which case you'll want to spit it out the moment you taste it anyway. On the contrary, microwaves don't sear the surface and leave burnt bits, which contain carcinogens, so microwaved food is not more but less carcinogenic than oven-baked or grilled food.
Nearly all chemical transformations in microwaved food occur because the food simply gets hot, just like it does in a conventional oven.
This is absolutely true. So do light bulbs, ordinary ovens, wood-burning fireplaces, and even human beings. The microwaves generated inside a microwave oven are radio waves with wavelengths from a millimetre to a metre, which carry far less energy per photon than even ordinary visible light. (Gamma rays, by contrast, have wavelengths down to 10-12m, short enough to cause tissue damage by ionization.) The reason microwave ovens are enclosed in wire mesh Faraday cages is because of the amount of microwave radiation, not anything inherently more dangerous about microwaves vs. any other kind of radiation.
Radiation leakage was a concern at one point, but the only way for a microwave oven to leak radiation is if there's a hole in its Faraday cage. And even if there is a hole in the microwave oven, the leaked radiation won't get very far — within a foot or two, it will have spread out so thin that it won't be any more harmful than the radiation from a cell phone. (Cell phone phobia is another matter entirely....) A hole is bad, but only for the exact same reason you shouldn't leave a conventional oven door open: it's a waste of energy and you could get burnt.
There is one kind of radiation microwave ovens freely emit into the environment: thermal infrared radiation. This is because they are at room temperature, and all objects at room temperature emit thermal infrared radiation. (Scary note: Your microwave oven emits thermal infrared even when it's unplugged!)
Reminder: never attempt to operate a microwave oven with a damaged door or a door that does not close completely.
Microwave ovens make food radioactive!
No, they don't.
Although many people refer to microwave cooking as "nuking" a piece of food, no nuclear processes are involved or induced. The only way to make a non-radioactive item radioactive is to bombard it with slow neutrons. While nuclear reactor cores emit neutrons, microwave ovens most emphatically do not.
Microwave ovens cook from the inside out
Not exactly. The microwave field penetrates a certain distance into food and no further; in meats, this is about two inches/5cm. Further cooking is done by heat generated from microwaving the outer parts of the food, migrating inward. However, as discussed above, some materials (such as dry pastry) are more transparent to microwaves than others (such as wet pie filling), making the food seem to be heated from the inside out.
They use microwaves at water's resonant frequency
Many folks, including some physics professors who should know better, repeat this claim, and this "fact" usually finds its way to textbooks, though likely as an oversimplification.
The claim is that microwave heating penetrates into the food rather than just heating the outside surface because the microwaves used by these ovens are tuned to exactly the resonant frequency of water molecules. Therefore their energy is absorbed by the water in the food and bypasses everything else.
Microwave ovens are not tuned to a specific "magic" frequency. Household microwaves produce radiation at 2.45 GHz. Absorption of microwave radiation by water takes place in a continuum of frequencies, with frequencies from 200 to 2.45 GHz being absorbed as much or more than at 2.45 GHz. The maximum is strongly temperature-dependent. The frequency of 2.45 GHz is chosen mainly to avoid interference with radio communications, but other frequencies can also be used, and large industrial and commercial ovens operate at 915 MHz instead.
Microwaves can penetrate to a depth of 1-3 cm into the food. If 2.45 GHz was the exact resonant frequency of water, the water on the surface of the food would absorb all the energy. This would be called — in less technical terms — "grilling".
It is true, though, that wet food absorbs microwave energy faster than dry food, since microwave radiation preferentially excites polar molecules such as water. Each material has its own loss function. Ice is largely transparent to 2.45 GHz microwaves, so melting frozen foods in a microwave risks thermal runaway — the sides melt and heat to boiling point while the center stays frozen solid. Ceramics such as ceramic plates and some types of plastics can be heated by microwaves without water being involved. There is no water in metal, yet it's notorious for heating rapidly in a microwave. To avoid uneven heating or to speed up the heating, you could try to spread water on your food and place a microwave cover on top to retain the moisture — the microwave will dry your food anyways, so that will get offset as well.
Microwaved water is bad for plants
This claim pops up from time to time: Water that's been boiled in a microwave and then cooled is somehow worse for your plants than water that hasn't been microwaved or was boiled on a conventional stove.
Mythbusters did a segment about this with 4 groups of plants: Plants given straight tap water, plants given tap water boiled in a microwave and cooled, plants given tap water boiled on the stove and cooled, and plants given no water. The result: All plants grew about equally well, except for the plants that were given no water and died. Truly an absolutely stunning result.
Masaru Emoto claimed that water that had been microwaved and frozen would produce ice crystals of a different shape than water that had not been microwaved before it was frozen. He claimed these crystals were just as malformed as the ones produced by yelling "Hitler!" (or projecting other negative thoughts) at the water before freezing it. Food Babe used this "fact" in an article titled "Why you should throw out your microwave," which mysteriously vanished from her website right before her first book was published.
So far as has been determined by rigorous controlled testing, there is no established link between microwave-heated water and membership in the Nazi party.
You can't brown a cake or pie in them
Actually, this one's true, unless you buy a browning plate which can be heated up to do the browning. 🙁
- Does microwaving food remove its nutritional value? — from cnn.com (The short answer: No, but make sure you only use microwave-safe cookware.)
- Anything new on the dangers of microwave ovens?, from The Straight Dope 28-Feb-2014
- Cooking in The Microwave Oven: Is It Safe?, a surprisingly well-informed article for a website with "wellness" in its name
- Roer, T.G. (2012). Microwave Electronic Devices
- Percy Spencer, Microwave Oven
- Technology Leadership. Raytheon Corporation
- Microwave Heating Fact Sheet, Washington State University
- Materials, What type of clay is used to make porcelain in China?
- "Microwave Ovens: The Curse of Convenience"
- Microwaves, Nazis, Cults, John F Kennedy, Anti-Semites and Christian Fundamentalists — Or cooking a dinner in 3 minutes flat
- Andjus, R.K. and Lovelock, J.E. 1955. Reanimation of rats from body temperatures between 0 and 1 C by microwave diathermy. J. Physiol
- For example: Why Did the Russians Ban an Appliance Found in 90% of American Homes?
- Microwave Myths (pdf)
- The Claim: Microwave Ovens Kill Nutrients in Food, New York Times, 17-Oct-2006
- The Hidden Hazards of Microwave Cooking, at health-science.com
- Kappe et al., Microwave Effects in Organic Synthesis-Myth or Reality?, Angewandte Chemie, vol. 52 no. 4 (21-Jan-2013), pp. 1088-1094.
- Radiation, irradiation and our food supply
- A 2.45 GHz photon carries 1.0132×10-5 electronvolts of energy. For comparison, visible light is roughly between 1.65 and 3.26 eV, or between 163000 and 322000 times more energetic.
- Do microwave ovens cook from the inside out?, on The Straight Dope, 4-Sept-2003