Regular ovens seem easy to understand. One can see the blue flames in a gas oven or the glowing red coils of an electric range. The entire oven is hot to the touch as heat penetrates both the food and the oven walls. But to the nonscientist, microwave ovens are mysterious. There is no fiery glow, and the oven stays cool, even as the food heats to boiling. It is as if microwaves were aimed only at the food.
Actually, though, microwaves are similar to the infrared energy that cooks food in a regular oven. Both are forms of electromagnetic radiation composed of streams of particles called photons. In a conventional oven, the burning of gas or the heating of a metal coil leads electrons in the gas or metal to give off infrared radiation for heating food. Microwaves are less energetic than infrared radiation, yet they are absorbed more quickly than water molecules. Water molecules respond readily and become excited because, like submicroscopic magnets, they have positive and negative ends which are easily charged up by a passing field of microwaves that is alternating rapidly—2.4 billion times a second. The charged water molecules are kept spinning by this action, which leads to friction, which in turn gives off heat, which dissipates to other molecules (e.g. food) that heat up and cook..
The key device in a microwave oven is the magnetron, a TV tubelike apparatus that works somewhat like a whistle. Just as a tiny clattering ball changes air vibrations in a whistle, so too a positively-charged metal strip with dime-sized dimples changes the pathway of electrons whizzing by and causes them to vibrate so they give off microwaves. The microwaves are then steered into the oven chamber by a metal "wave guide" and "stirred" to spread them evenly throughout the oven. The oven is made of metal and the metal reflects the microwaves back toward the food. Thus the microwaves stay inside and the oven remains cool. Though glass permits the passage of microwaves, the metal grill behind the glass door retards their passage. Visible light, however, has a shorter wavelength and can easily pass through the grill allowing us to see what is cooking.
Can microwaves be used to kill bacteria? Most probably, so long as water molecules are present. For a kitchen sponge the microwave oven is effective for lowering the bacterial content. For a relatively dry cutting board, however, there is little water available, and the bacteria may not offer much of a target for the radiations. Heating in boiling water might be a more effective method for killing microorganisms.
