Percy Spencer was a self taught engineer was testing on his magnetron vacuum tube on a research project pertaining to radar.  That was when the starting point of a great invention.  He observed that his candy peanut bar in his pocket was melting!  What could this happen?  This really interested him and being an inquisitive scientist, he would not give up putting this aside.  He tested some popcorn kernels by putting them near the tube and CRACK!  The popcorn popped all over the place.  Having a hypothesis, he tried it again with an egg.  To his anticipation, it exploded and slashed its yoke all over.  YES!  It must be the exposure of low density microwave energy!  Further to his investigation, Spencer transformed a metal box into an appliance we nowadays called the MICROWAVE OVEN by feeding in microwave energy - that is to block any form of energy exiting the box, maintaining a higher electromagnetic field density within.  So, Spencer blueprinted the basis of this great invention.

How put this into real practice?

It was not until late 1946 that the Raytheon Company proposed to use it for cooking.  Placing on in a Boston restaurant was the first test attempted.  After a series of probations and improvements, finally, the first microwave oven was born.. It was huge, tall with a standing height of 5 1/2 feet and heavy, with a weight of 750 pounds.  Furthermore, it cost $5000 each.  Installations for plumbing are needed since the magnetron tubes are required to be water - cooled.  For its convenience and efficiency, the first home use microwave oven was introduced by Tappan, costing $1295 in 1952 - 1955.  15 years later, an even modified version was introduced - a 100 volt, " countertop, domestic" oven, smaller in size, and priced under $500 was widely welcome.

So, how does it really work?

A microwave oven uses microwaves to heat food. Microwaves are radio waves.  In the case of microwave ovens, the commonly used radio wave frequency is roughly 2,500 megahertz (2.5 gigahertz). Radio waves in this frequency range have an interesting property: they are absorbed by water, fats and sugars. When they are absorbed they are converted directly into atomic motion - heat. Microwaves in this frequency range have another interesting property: they are not absorbed by most plastics, glass or ceramics. Metal reflects microwaves, which is why metal pans do not work well in a microwave oven.

In microwave cooking, the radio waves penetrate the food and excite water and fat molecules pretty much evenly throughout the food. There is no "heat having to migrate toward the interior by conduction". There is heat everywhere all at once because the molecules are all excited together. There are limits of course. Radio waves penetrate unevenly in thick pieces of food (they don't make it all the way to the middle), and there are also "hot spots" caused by wave interference, but you get the idea. The whole heating process is different because you are "exciting atoms" rather than "conducting heat".

In a microwave oven, the air in the oven is at room temperature, so there is no way to form a crust. That is why foods like "Hot Pockets" come with a little cardboard/foil sleeve. You put the food in the sleeve and then microwave it. The sleeve reacts to microwave energy by becoming very hot. This exterior heat lets the crust become crispy as it would in a conventional oven.

            --www.howstuffwork.com

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