Warning! Microwave ovens carry high-voltage! Do not attempt repairs without taking the proper precautions. We do not recommend home repair of this appliance. Serious risk of injury and the risk of death come with opening up a microwave oven. There is a high-voltage capacitor inside this appliance. Warning!
Microwave ovens use microwave energy to cook or heat food. A microwave passes microwave radiation, at a common frequency of 2.45 GHz, which is equal to a wavelength of 12.24 cm, through foods. This radiation is really just a high frequency radio wave. The fat, water, and other molecules in the food act as electric dipoles. A dipole has a negative charge at one end, and a positive charge at the other. The dipoles try to align themselves with the alternating electrical field generated by the microwave radiation by rotating themselves. This causes a lot of bumping between the molecules, which creates heat by friction.
A common misperception is that food cooks from the inside out with microwaves. This is not true. The microwaves actually penetrate deeper initially, up to a couple of centimeters deep, than does the heat from convection and radiant heat cooking. This makes it appear that the food gets cooked from the inside out.
The cooking area is enclosed by a Faraday cage, which keeps the radiation safely inside the oven, and away from people and pets. The mesh you see on the door allows visible light to pass through, letting you watch the food cook, while blocking the microwaves. This can happen because the frequency of the microwaves is much larger than that of visible light. The legal limit of microwave leakage for new microwaves in the United States is 1 mW/cm² at 5 cms. This is way below the exposure limits thought to be harmful to humans.
When cooking or reheating foods in a microwave oven, especially a convection microwave oven, you need to allow at least 1″ clearance between the food containers and all the walls and door. This will allow proper air circulation for optimum microwave performance.