m i c r o w a v e s is why , The extent to which microwaves are reflected varies with the type of metal. For example, aluminum and stainless steel reflect microwaves, while certain compositions of cold-rolled steel will absorb microwave energy to some extent. However, for all practical purposes, it may be said that metal reflects microwave energy.
When metallic objects are indiscriminately placed in a microwave oven cooking cavity, the energy pattern becomes disrupted and distorted. This condition produces arcing. When two metal objects, such as a metal bowl and the [metal] cavity wall, are placed in close proximity and subjected to an intense field of microwave energy, arcing will result. Arcing occurs because the air between the two metallic objects becomes electrically charged, just as the air between a thundercloud and the earth becomes charged or ionized. This ionized air becomes an electrical conductor, and electric current then leaps the gap like a small bolt of lightning.
However, lightning only lasts for an instant because it discharges or neutralizes the ionized air, but an arc in a microwave oven will continue, to a greater or lesser degree, as long as the microwave energy is applied. At the very least, this can cause marring or pitting of involved surfaces, and at worst, can burn a hole right through the cavity wall.
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