How a Microwave Oven Works
Microwaves are like light waves. Microwaves can pass through some things, are absorbed by some things and reflected by other things. Light can pass through clear glass, is absorbed by dark objects (like black upholstery in a car) and is reflected by light objects (like white upholstery).
When microwaves pass through something, like glass, they do not have an affect on it. When they are reflected, such as by metal, they also have no affect on the object. But when they are absorbed, like light being absorbed by the black upholstery, heat is generated. One difference with microwaves is that they penetrate food and so cooking occurs throughout, whereas light waves do not penetrate and so heat from the outside inward. *Note that glass and metal actually do absorb some microwaves and heat up because the glass and metal we use is neither perfectly transparent nor perfectly reflective. Most of the heat they pick up in the microwave is through heat conducted from the food itself.
The unique thing about microwaves, and what makes them useful for cooking, is that they happen to be absorbed by water, fat and sugars. Most foods are comprised of some combination of water, fats and sugars. So, because foods absorb microwaves, they get hot, much like black upholstery absorbs light and gets hot. Microwaves pass through glass and some types of plastic generating little or no heat.
The Mechanics of a Microwave
Electrical current travels through the controls, a fuse, a thermal cutoff, a safety interlock on the door, to a high-voltage transformer and then on to the magnetron. The magnetron uses very high voltage and for this reason, we do not recommend that you open your oven to make repairs. This voltage remains in a capacitor even when the oven is unplugged. Because of the deadly voltage, the high cost of parts and the low cost of a new oven, replacement is very often a better option than repair.
The magnetron requires the high voltage to generate microwaves. The waves are then focused into the cooking chamber. The interior surfaces, including the mesh over the window, are metal and reflect the microwaves so that they penetrate the food from all sides to heat the food evenly. A stirrer and/or a turntable are used to further improve the even absorption of the microwaves.