There is so much anxiety among home cooks about microwave ovens that you’d think they were kitchen-sized nuclear reactors.
The situation is not helped by some authors of food books, who seem not to know the difference between microwaves and radioactivity. Yes, they are both radiations, but so are the television radiations that bring us vapid sitcoms. It’s hard to say which are more to be avoided.
Microwaves are waves of electromagnetic radiation just like radio waves, but of shorter wavelength and higher energy. (Wavelength and energy are related; the shorter the wavelength the higher the energy.)
Electromagnetic radiation consists of waves of pure energy, traveling through space at the speed of light. Light itself, in fact, consists of electromagnetic waves of even shorter wavelength and higher energy than microwaves. It’s the specific wavelength and energy of a radiation that gives it its own, specific properties. Thus, you can’t cook food with light and you can’t read by microwaves.
Microwaves are generated by a kind of vacuum tube called a magnetron, which spews them out into your oven, a sealed metal box in which the microwaves continually bounce around as long as the magnetron is operating. Magnetrons are rated by their microwave power output, which is usually from 600 to 900 watts. (Note that this is the number of watts of microwaves produced, not the number of watts of electricity that the appliance uses, which is higher.)
But that doesn’t tell the whole story. The cooking power of a microwave oven, and hence how fast it will do its chores, depends on the number of watts of microwaves there are per cubic foot of space in the box. To compare ovens, divide the microwave wattage by the number of cubic feet.
For example, an 800-watt, 0.8-cubic-foot oven has a relative cooking power of 800 ÷ 0.8 = 1000, which is pretty typical. Because different ovens have different cooking powers, recipes can’t be specific about how long any given microwaving operation should take.