From the first food on an open fire to the inefficient big ovens in our kitchen today, cooking was previously a matter of heating from the outside in. By process of conduction, heat is introduced on the outside and has to migrate toward the center of a roast or bread pan. In the case of potatoes or a Thanksgiving turkey, this can take a long time.
Microwave energy is a relatively low frequency electromagnetic pulse creating an intense beam of particles at about 2500 megahertz. Bombarding food in an oven, these energized particles are easily absorbed by water, sugars and fats. Once absorbed, they are immediately transformed into heat.
Conveniently, these waves are not absorbed by plastic, glass or ceramic materials, making containers made from these materials ideal for use in ovens. Metal reflects the particles and is therefore not conducive for microwave heating.
Because microwaves penetrate food substances at the atomic level, they stimulate all of it at once. Every particle is “shaken up” and heated simultaneously. There is no heat moving from the outside in requiring an elapsed time for the convection, but it all happens quickly, greatly reducing cooking times.
Even while it is cooking, the air in a microwave oven remains at room temperature, so there is no way to form a crust. Hence microwavable pastries sometimes come with a little foil or cardboard sleeve. The sleeve reacts to microwave energy by becoming very hot and this exterior heat creates the crispy crust you expect in a conventional oven.