A microwave oven contains a device called a magnetron, which produces the microwaves that cook the food you place in the oven. Mircrowaves are a form of low-frequency electromagnetic radiation with a host of applications that include cell phone transmission. Unless your microwave oven is broken, though, you don't have to worry about being exposed to the radiation the magnetron generates -- the metal casing of the oven compartment keeps it safely confined to the oven.
A Form of Infrared Radiation
Microwaves are vibrations of the electromagnetic filed; other examples of electromagnetic energy include visible light, the X-rays and gamma rays generated by stars, and the radio waves that scientists use to look at those stars. The electromagnetic spectrum ranges from gamma rays, which have the shortest wavelengths and highest frequencies, through visible light to radio waves at the other extreme. Violet has the highest frequency of visible light and red the lowest, so scientists call high-frequency radiation "ultraviolet" and low-frequency radiation "infrared." Microwave radiation, with frequencies around a few billion cycles per second, is in the infrared part of the spectrum. The radiation emitted by a microwave oven magnetron is typically around 2.5 GHz.
Microwave Generation in an Oven
When you plug in your microwave oven and turn it on, the magnetron -- which is a type of vacuum tube -- generates radiation via electrical and magnetic processes. The magnetron is oriented in such a way that the radiation enters the oven compartment through a waveguide. The waves bounce back and forth between the walls of the oven compartment -- which are designed to reflect it -- and create a standing wave pattern inside the oven. If you were to open the oven door while the magnetron was operating, some of this radiation could escape, but most ovens have a safety feature that shuts off the magnetron when you open the door.
Microwaves Heat Water
The secret ingredient for cooking with microwaves is water. Your food must contain moisture -- fat will do -- or it won't cook. Water molecules are bipolar, which means they are like tiny magnets, and they change orientation to align with the microwaves, which have rapidly fluctuating electric and magnetic fields. The rapid vibrations generate heat as the water molecules rub against each other, and it's this heat that cooks the food. Microwaves won't heat metal, porcelain, ceramic or any other dry substance. You can verify this for yourself by placing a bowl of dry baking powder in a microwave oven. It will remain cool unless you add a small amount of water.
Microwave Safety Tips
Many cell phones and cell phone towers use microwaves of the same frequency as those generated in a microwave oven, but the difference is in the amount of power. Cell phone radiation is of the order of a few hundred milliwatts, while the radiation in a microwave oven is between 700 and 1,000 watts. Fortunately, the radiation is contained if the walls aren't damaged -- if they are, replace the oven. Microwaves won't heat metal, but they do create excess charge on metal surfaces, which is why you should avoid putting metal objects or aluminum foil in a microwave oven. The buildup of charge causes arcing that can start a fire.
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