<Tr> <Td_colspan="2"> Problems playing this file? See media help . </Td> </Tr> <P> A microwave oven (commonly referred to as a microwave) is a kitchen appliance that heats and cooks food by exposing it to electromagnetic radiation in the microwave frequency range . This induces polar molecules in the food to rotate and produce thermal energy in a process known as dielectric heating . Microwave ovens heat foods quickly and efficiently because excitation is fairly uniform in the outer 25--38 mm (1--1.5 inches) of a homogeneous, high water content food item; food is more evenly heated throughout than generally occurs in other cooking techniques . </P> <P> The development of the cavity magnetron in the UK made possible the production of electromagnetic waves of a small enough wavelength (microwaves). American engineer Percy Spencer is generally credited with inventing the modern microwave oven after World War II from radar technology developed during the war . Named the "Radarange", it was first sold in 1946 . Raytheon later licensed its patents for a home - use microwave oven that was first introduced by Tappan in 1955, but these units were still too large and expensive for general home use . The countertop microwave oven was first introduced in 1967 by the Amana Corporation, and their use has spread into commercial and residential kitchens around the world . In addition to their use in cooking food, types of microwave ovens are used for heating in many industrial processes . </P> <P> Microwave ovens are popular for reheating previously cooked foods and cooking a variety of foods . They are also useful for rapid heating of otherwise slowly prepared cooking items, such as hot butter, fats, and chocolate . Unlike conventional ovens, microwave ovens usually do not directly brown or caramelize food, since they rarely attain the necessary temperatures to produce Maillard reactions . Exceptions occur in rare cases where the oven is used to heat frying - oil and other very oily items (such as bacon), which attain far higher temperatures than that of boiling water . </P>

When did microwaves become a common household appliance