A typical microwave oven has an optoelectronic device, called, magneton which holds a thermionic electron emitter (cathode) and an electron collector (anode), both fitted into an evacuated cylinder under a magnetic field. The ejected and accelerating electrons take a curved path to reach the destination (anode) while transmitting microwave radiation. It is this radiation that is dispersed into the cooking compartment by appropriate electromechanical design.
Substances, with polar molecules such as water, alcohols, oils, etc, absorb microwave radiation to elevate the rotational movements of their molecules. When foodstuffs to be cooked by microwave oven are kept in this kind of kettle, the majority of the microwave power reaches the water and tends to hasten the molecular rotations. But, owing to the matrix and intermolecular ligaments, water molecules experience restricted rotation and dissipate the absorbed microwave optical energy into heat out of molecular frustrations. It is this heat that causes faster cooking of the food stuffs.
When we heat a cup of water alone in a microwave oven, the water gets all the microwave energy and becomes hot.
Though designed to distribute the microwave energy homogeneously (isotropic) into the cooking compartment, the top regions, usually receive a slight excess.
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