Microwaves possess three basic characteristics:

1. Just as sunlight shines through a window, microwaves pass right through some materials. Materials such as glass, paper, and plastic are transparent to and generally unaffected by microwaves.
2. Microwaves are reflected by metal surfaces, much as a ball would bounce off a wall. The metal walls of the cooking space actually form a cavity resonator. In other words, the enclosure is designed to resonate the microwaves as they are radiated from the magnetron tube. The principle of resonance may be illustrated using sound waves. When a piano key is struck, it produces sound vibrations or sound waves. Sometimes a note is played on a piano, and an object across the room, perhaps a wineglass, can be heard vibrating and producing the same sound. This is the result of resonance. The resonating characteristics of the wineglass are the same as those of the piano string. Therefore, the wineglass is in tune, or in resonance, with the sound wave produced by the piano string. In the same way, the cooking cavity of a microwave oven is designed to be in "tune" with the resonant characteristics of the microwaves.
   • Metal racks are physically proportioned so as not to disrupt the energy pattern.
3. Microwaves penetrate and are absorbed by some substances, primarily food products. As the energy penetrates the food, its power is gradually absorbed, or lost, to each successive layer of molecules. The rate of energy loss and depth of penetration vary with the depth, density, chemical properties and temperature of the food. However, on the average, the power is cut in half about every three-quarters of an inch of penetration. Therefore, since the intensity of the electromagnetic field is less at the center of the food than at the surface, the molecules closer to the center of the food do not feel the full effect of the energy.

Friction Produces the Heat That Cooks the Food

To illustrate this third characteristic, notice the cooked turkey below. The waves of microwave energy are cycling above and below a horizontal baseline. The half cycle below the baseline possesses negative properties, and the half cycle above the line is correspondingly positive. Basically, the effect of this wave, as it alternates between positive and negative, would be like a magnet flipping back and forth.  All liquids and food products, such as this turkey, are made up of molecules. These molecules have positive and negative particles, so they tend to behave like microscopic magnets. As the positive half cycle of the microwave penetrates the food, the negative particles of the molecules are attracted and attempt to align themselves with this positive field of energy. Then, when the microwave energy alternates to the negative half cycle, the opposite occurs -- The negative particles are repelled and the positive particles are attracted, causing a flipping motion (actually, this reaction is the movement of the particles within each molecule, so, technically, they reverse polarity). This might be compared to a room full of people trying to run back and forth, from one side to the other. Obviously, there would be a lot of bumping, rubbing, agitation, and friction.
Now, consider that the actual frequency of the RF energy used in microwave ovens is 2450 million cycles per sec! Moreover, consider that within the course of one of those cycles, the molecules would actually change their direction (polarity) twice - once for the positive half-cycle and once for the negative half-cycle. This red-hot rate of vibration causes tremendous friction within the food, and - just as rubbing your hands together makes them warm - this friction produces heat. So the heat is produced directly in the food, but the food is not cooked, as is commonly believed, from the inside out. Actually, the cooking begins just beneath the outer surface and from there inward and outward, with the majority of the energy being expended in the outer layers. The rate and degree of heating depend on the depth and density of the food, as well as its ability to conduct heat. Because the microwave energy is changed to heat as soon as it is absorbed by the food, it cannot make the food radioactive  or contaminated. When the microwave energy is turned off and the food is removed from the oven, there is no residual radiation remaining in the food. In this regard, a microwave oven is much like and electric light that stops glowing when it is turned off.