Microwave ovens are complex heating and cooking devices that have been standard appliances in most U.S. homes since the 1970s. When used properly, microwave ovens are safe and effective. The magnetron, located between the shell and inner cooking chamber of the oven, produces electromagnetic waves that agitate the water molecules in food and liquids. Friction from the repolarization of the molecules generates heat, which cooks the food. The waveguide is a thin sheet of mica on the inside of the port of the cooking chamber. It transmits microwave radiation while preventing steam and food particles from interacting with the sensitive electronic components of the microwave oven.
Typically, the microwave oven's cooking chamber is not centered to its shell. Behind the control panel are the electronic components, including the high-voltage transformer and the magnetron. When the oven door is open, you can see into the cooking chamber. On the inner wall of the chamber, on the same side as the control panel, you will see a thin sheet of what appears to be plastic or thin cardboard. This is the waveguide.
The waveguide is a thin semipermeable sheet of mica. The waveguide is able to transmit microwave radiation from the magnetron to the cooking chamber. Its unique physical characteristics prevent vapor and food particles from entering the space between the cooking chamber and the oven's outer shell, where sensitive electronic components are located.
With proper use, the waveguide will likely last the lifetime of the microwave oven. No lateral or vertical stress is exerted on it. The mica waveguide, however, is brittle and may be easily broken if dishes are not carefully placed in the cooking chamber. A broken waveguide should be replaced before continuing operation of the oven.
If the waveguide is broken or absent, steam and vaporized food particles will accumulate on the oven's inner electronic components. This may cause corrosion of and electrical arcing in the circuit boards. In either event, the microwave oven will eventually or immediately cease to function.