You are right -- microwave ovens can ruin food. They can cause potatoes
and eggs to explode, and can cook foods non-uniformly, making them less
appealing than foods that are cooked more traditionally. For more
information on how microwaves cook foods, please see our answer
and this great Questions and Answers site at the University of Virginia
Microwaves are rapidly oscillating electric and magnetic fields,
confined inside the metal box of the oven. The metal walls of the oven
act like mirrors which reflect microwaves back to middle of the box.
The reason the metal walls act like mirrors is because they are good
conductors of electricity. See this page
for a description of why electrical conductors make good mirrors. The
metal walls of a microwave oven are usually either painted or lie
behind a layer of plastic so that they can easily be cleaned. If the
resistivity of the metal is low, then the metal will not heat up when
it reflects the microwaves and will not be damaged. Since the metal
walls have some resistance, most microwave ovens include cooling fans
to cool down the microwave source and possibly also the metal walls so
that heat deposited in the walls will not damage the oven.
may be possible to damage a microwave oven by leaving it cooking on
high power for a long time (particularly if there is no food in it).
But the amount and rate of damage (if any) depends on how well the
cooling fan works and whether the oven is protected in other ways (some
may even have temperature sensors that shut down the oven before the
components get too hot, or they may have a fuse protecting the
capacitor in the microwave source). As long as they don't get too hot
to damage anything, the metal parts should not wear out. The cooling
fan may wear out after a long time, not because of microwaves but
because of dust, grit, or loss of lubrication.
(published on 10/22/2007)