The rapidity with which microwave ovens cook is due not so much to the total power delivered into the oven as to the fact that the microwaves penetrate the outer layers of the food and deliver their energy within the food itself. In both conventional and microwave ovens, food cooks from the outside in. Because microwaves reflect from the inside walls of the oven, it is possible to set up standing waves within the oven, similar to the way standing sound waves are set up in a pipe. The nodes and antinodes correspond to cold and hot spots, which lead to uneven cooking. Food enclosed within metal containers does not cook because the metal shields the food from the microwaves.
Caution is appropriate when using microwave ovens.
The microwave power output of typical household ovens is about 500 W, a level high enough to cause burns. However, a level of microwave power that is too low for cooking may cause serious damage to humans. Therefore, standards have been set to limit how much radiation may escape from the oven. For safety, a conducting plastic seal ensures that microwave energy does not leak out around the door. For additional safety, an electrical interlock shuts off the oven when you open the door.