Reprinted from http://amasci.com/weird/microexp.html#other

Q: Do Microwave Ovens cook from the inside out?

A: Nope. Food is partially transparent to the radio waves, so the energy is able to shine through it. But at the same time the waves are partly absorbed by the food. Usually most of the heat is produced in an outer layer about an inch thick. So, large pieces of meat will be quickly cooked to a depth of about an inch, while the inside portions are cooked by heat conduction, just like in a conventional oven. This effect is different for different foods of course. If a food is mostly water, then only the outside inch cooks at all. If a food contains both air and water (like bread, cake, etc.,) then the radio energy penetrates all the way through, and the food gets heated everywhere, even deep inside.

Q: If I put a fork in the Microwave, will it destroy the oven?

A: Nope, this is a myth, but it has some roots in reality.

In order to safely use metals inside a microwave oven, the cook has to learn numerous complex and mysterious rules in order to avoid fires and undercooked food. For example, thin metal will heat up fast in the oven, and may cause fires. The famous problem of the staple in the paper popcorn bag comes to mind, where the staple heats up and sets fire to the bag. And if a metal object in the oven is lightly touched to another one, or touched to the metal wall of the oven, an electric arc might ignite at the contact point. If not stopped it can set fire to the oven. In the higher power ovens when the amount of food is small, sharp points on metal objects can initiate a corona discharge, a "Saint Elmo's Fire," which behaves the same as a flame and can set fire to the food and the oven if allowed to continue for long.

So, it's much easier to totally ban the use of metals in microwave ovens. The alternative would be to send everyone to school to learn the complicated rules!

Q: What about the dangerous radiation?!!

A: Microwave ovens don't use "radiation." Instead they use radio waves.

Usually when we talk about radiation we mean radioactivity or "Ionizing Radiation," gamma rays and high energy subatomic particles. Microwaves aren't high energy, the microwave photons less energetic than visible light. Yes, if the source of microwaves is a very "bright" source, it can cook food. If normal light is very bright, light can also cook food (think of those solar ovens used by campers.)

Note that technicians and scientists talk about "microwave radiation" in the same way that they talk about "optical radiation." Optical radiation is just another word for VISIBLE LIGHT. They're using the word "radiation" to mean "waves." In this sense light is radiation, the warmth of an electric heater is radiation, even radio waves are radiation. And when you speak, some "acoustic radiation" comes out of your mouth. But radioactivity is an entirely different thing. Microwave radiation doesn't sterilize your reproductive organs like high-energy X-rays would. (An oven with a damaged door can only cook you!)

Speaking of that, how can you tell if your microwave oven is safe? There's one simple way to detect a major leak microwave. Get a fluorescent tube and hold it near the edges of the oven's door when the oven is on. Microwave leakage will make the bulb glow. (Do this in a darkened room so you'll see the slightest glow.) This works much better if there is nothing at all inside the oven. If you own an older oven (pre 1980s) you might want to include a glass of water. However, this method only shows the larger leaks. Your oven might be leaking too little, so it won't light a fluorescent tube. To catch even the smallest leakage, you need a "microwave leakage detector." Search the web for these. I've sometimes seen them on eBay for $10.

Q: Aren't these ovens tuned to a special frequency so they only heat water?

A: No. The usual operating frequency of a microwave oven is far below the resonant frequency of water vapor... and liquid water doesn't have a resonant frequency. Also, the radio energy in a microwave oven can heat many other substances besides H2O. Water isn't special. For example, drops of grease on a plastic microwave dish will be heated far hotter than 100C, and this causes the mysterious scarring which frequently occurs on plastic utensils. Any molecule which is "polar" and has positive and negative ends will be rotated back and forth to align with the changing e-field of the radio waves in the oven. The vibrating electric field of the radio waves vibrates the oil and water molecules and any other polar molecules within the food. Jostling molecules equals heat! Microwave ovens have difficulty melting ice, presumably because the water molecules are bound together and cannot be easily rotated by the e-fields.

If liquid water had a narrow resonant frequency, and if the oven was tuned to this frequency, then the water would be far more opaque to the wave energy. The water in the food's thin surface would absorb all the energy, and only the outside surface of foods would be heated. The thin outer surface of meat would become a blast of steam, and the inside would remain ice cold. [Perhaps the oven would act like a normal electric oven, charring the outside but only heating the inside very slowly.] But because water does not resonate with the microwave frequency, the waves can travel an inch or so into the meat before being absorbed. Microwave ovens heat a thick layer of meat, not a thin layer.

Another note: single H2O molecules have a sharp resonant frequency, but liquid water does not. In order to have a distinct resonance, a water molecule must be alone in space, not bound to billions of identical neighbors. The bonding to neighboring water molecules spoils the sharp resonance and greatly widens the frequency band. Liquid water has a huge, wide absorption band, not a single resonant frequency. In other words, water absorbs all short radio waves. Typical microwave ovens don't even use the best frequency. They should be up around 10GHz frequency rather than the usual 2GHz, but that would make the microwave tube more expensive.

Q: Don't you need distilled water to cause superheating and "explosions?" Impure tap water only boils, it won't explode.

Ummm. What?

Of course impure water can "explode." That's why people get scalded; they re-heat their coffee in microwave ovens and then quickly add sugar. Coffee is extremely impure water! To cause "coffee explosions," you don't need special pure water. All you need is water that lacks micro-bubbles.

It seems that some recent TV show tries to prove that only pure water can superheat and "explode." This is wrong. Probably the creators of that show did not know an important fact: while crystals will grow upon solid nucleation centers, the gas bubbles in cola and in boiling pots grow upon microscopic seed-bubbles. You can't grow crystals unless you have a solid microscopic seed. And you can't boil water unless you have seed-bubbles present. It doesn't matter how filthy your water is, or how many crystal nucleation centers it contains, if it lacks seed-bubbles then it will not boil normally, instead it will superheat.

Clearly this has nothing to do with distilled water. In fact it's very easy to boil distilled water normally, without explosions. First chill and shake up your bottle of distilled water thoroughly. And it's easy to make "impure" water explode. If tap water has had all the bubbles cleared out by earlier boiling or by simply sitting for days in an open container, then it will superheat, and when sugar is dumped in, it will go DOOSH and spray all over.

Q: I want to buy a microwave space-heater!

Heh. Me too. Why aren't they available? One main reason springs to mind: in your eyes, the cornea and internal lens are very sensitive to heating, since they have no blood vessels to keep them cool. Intense radio waves can give you instant cataracts, so a microwave heater would require that we use special goggles to prevent blindness. Another problem: a normal heater makes your skin surface hot, while a microwave heater makes a thick layer of flesh get hot. With a normal heater, you can yank your hand away when you feel pain, and your thin skin cools down almost instantly as the heat migrates to fill a thicker layer. With a microwave heater, you might receive internal burns at the same time that you feel pain on your skin. And when you yank your arm away, then the thick layer of heated flesh wouldn't cool instantly, causing even more tissue damage. And last, a microwave heater is an intense electrical source, and it would probably destroy any radio, cellphone, PDA, or laptop that came near. Lawsuit city!

Q: Doesn't microwave energy lower the food's nutrition?

I don't know.

On the other hand, I've heard lots of crazy rumors along the lines of "microwave energy turns food into slow poison." Maybe it's true, or maybe all the rumors are just some BS made up by bored storytellers. However, because these crazy rumors exist, we must be on guard against believing them, and we should only trust information if we can get it from people who are up front about where the information came from in the first place. Anything else, and we'd end up believing the liars who have fun by starting rumors, hoping their rumors will catch fire and scare huge numbers of victims.

Just follow the same rule that you follow for crazy spam emails that give you all sorts of dire warnings about various topics. The rule: if the warnings were real, wouldn't it be dead easy for the original email author to include several URLs pointing to many articles about the danger? So, if an email doesn't link to real live websites, it's almost always a false rumor being spread by hoaxers. And with any health warning, if warnings about microwaves don't link to news articles or perhaps actual medical journals, almost certainly the author is passing on third-hand rumor rather than a story about a genuine hazard.

Below are a few of my own rumors! With no links to research or online news!

:)

I vaguely recall seeing something in the papers long ago about microwave ovens harming food vitamins. But I don't remember if they said that microwave-cooking is worse than REGULAR BOILING. I don't know if they said that the problem comes from simple overcooking rather than from any weird stuff with radio waves. I've always heard that eating raw vegetables is best, and cooking destroys vitamins. What happens when you feed people on overcooked or canned food for months? If somebody says that microwave cooking is much MUCH worse than a canned food diet, I'd like to find out WHY they say that. Maybe they're really just warning us about boiled vegetables, but trying to make out like the problem is with the microwave ovens rather than with the boiling. Microwave ovens cook food deeply and quickly, so if you boil your vegetables for five minutes on a stove, they'll still be green and crunchy, but if you boil your veggies for five minutes in a microwave oven, they'll be way overcooked. With microwaves, it's hard to cook only the outside surface of the vegetables. (Now that I mention it, even boiling water doesn't do such a great job. It's better to stir-fry veggies so the inside is almost raw but the outer layer is cooked.

If we need to be warned about any cooking, the warnings must be precise. And fake warnings themselves aren't innocuous. Don't forget, it was a rumor-storm of fake warnings about poisoned candy and razor blades in apples that ruined Halloween trick-or-treating in the US for decades years.

Another issue: microwave cooking unsafe... COMPARED TO WHAT? Life is unsafe. People die from taking showers or stepping on toys, not from eating microwaved food. Think a minute. When you eat browned meat or even baked bread, the browned parts are full of heat- shredded biomolecules which form all kinds of NASTY CARCINOGENS. Yet they seem not to harm people very much (perhaps we've all gotten used to the stuff over the millenia because we've been eating charred food ever since humans discovered fire.) As I understand it, browned food is a dark secret of cancer studies, and if they applied the current FDA regulations to normal foods like bread or BBQ, the government would have to ban cooking. They've been "grandfathered in." Yet perhaps the carcinogens in browned food do occasionally cause things like colon cancer? If they do, then perhaps microwave cooking is much safer than normal ovens, since microwave cooking is akin to steaming, and it's almost impossible to force your microwave oven to shred the molecules on the outside of your steak into tasty brown carcinogens.

Does this mean that microwave ovens are health-food devices? Maybe stores should've been selling microwave ovens next to the carrot juicers and wheat-grass kits? I don't know. First I'd want to know if anyone did any microwave-cooking nutrition studies at all, even with lab animals. And as a control, compare the results against such common health dangers like going swimming or (gasp) DRIVING A CAR... or at least compare it against baked bread and fast food hamburgers.

Q: Corona discharges? High voltage? Balderdash! The watts per cm^2 is too small!

A: Wrong, melted pyrex breath! You're thinking about cooking huge roasts. What happens with EMPTY microwave ovens?

Yes, a typical oven's output might only be 1000 watts or so. And yes, 1000W spread over a few hundred square centimeter does not produce strong e-fields. The fields will only be a few hundred volts per cm. (For corona discharge we need around 30,000V/cm.) But you're forgetting something important: the voltage rise for resonant circuits. If there's a huge roast (a significant electrical load) then... 1000W leaves the magnetron, and 1000W is absorbed by the meat. There's a 1000W energy flow between magnetron and food, and the max. e-field inside the oven stays low.

On the other hand, if there's no food in the oven, then the 1000W bounces back and forth, yet the magnetron still puts out more energy. This adds to the waves already there. It's like wiggling your hand in a full bathtub: energy is stored as standing waves, and the waves build up higher and higher until frictional losses finally halt their growth. Inside an empty microwave oven you might have 50,000 watts in one direction and 49,000W in the other (with the magnetron supplying the 1000W difference.) The interior of the oven is a resonator, and the peak wattage within that space can become HUMONGOUS, since it only depends on the "Q" factor of the resonator; it depends on the "friction" of the system. Q is high, so huge wattage gives huge voltage. If there's no food and no glass of water in the oven, then the e-fields become intense, and the tiniest burr on a metal object can trigger the formation of a large "Saint Elmo's Fire" which consumes hundreds of watts of RF energy and resembles a blowtorch.

In fact, the empty chamber of a microwave oven is very much like the secondary coil of a Tesla Coil. The only major difference is the operating frequency. RF energy is injected into the resonator, and the output voltage rises and rises until finally the conductors get hot (or until finally an electric arc breaks out somewhere.) Tesla coil secondaries do this. Microwave ovens do this too. With nothing in the oven chamber, either the metal walls and glass parts get very hot, or an electric arc bursts forth from a sharp metal point somewhere inside the oven cavity.