Microwaves are defined to be waves having frequency greater than 1 GHz. While some books put the upper limit of 30 GHz to microwaves, some literature consider the upper limit to be up to 300 GHz. At such high frequencies, the behaviour of the devices used commonly is no longer predictable by general rules and new design issues need to be considered. Even short leads of devices act as inductances and may possess capacitances.
A magnetron emits microwaves at 2450 MHz λ = 12.24 cm A microwave is a Faraday cage, so waves can't escape openings smaller than 6.12 cm. Microwaves work by bombarding atoms from all sides with an electric field. This causes the dipole to keep realigning itself and the food heats up. Most effective on water, metal deflects microwaves. Containers survive because they do not absorb many microwaves relative to the food.