This is the first of two articles about the physics of microwave ovens. This
article deals with the generation of microwaves in the oven and includes the
operation of the magnetrons, waveguides and standing waves in resonant
cavities. It then considers the absorption of microwaves by foods, discussing
the dielectric relaxation of water, penetration depths of electromagnetic
waves in matter and, in considering the possible chemical changes during
the microwave heating, multi-photon ionization or dissociation.