Water, fat, and other substances in the food absorb energy from the microwaves in a process called dielectric heating. Consumer ovens usually use 2.45 gigahertz (GHz)-a wavelength of 12.2 centimeters (4.80 in)-while large industrial/commercial ovens often use 915 megahertz (MHz)-32.8 centimeters (12.9 in). Microwave ovens use frequencies in one of the ISM (industrial, scientific, medical) bands, which are reserved for this use, so they don't interfere with other vital radio services. Microwaves are a form of non-ionizing electromagnetic radiation with a frequency higher than ordinary radio waves but lower than infrared light. PrincipleA microwave oven heats food by passing microwave radiation through it. However, additional heat sources can be added to microwave ovens, or into combination microwave ovens, to produce these other heating effects, and microwave heating The boiling-range temperatures produced in high-water-content foods give microwave ovens a limited role in professional cooking, since it usually makes them unsuitable for achievement of culinary effects where the flavors produced by the higher temperatures of frying, browning, or baking are needed.

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. Unlike conventional ovens, microwave ovens usually do not directly brown or caramelize food, since they rarely attain the necessary temperatures to produce Maillard reactions. They are also useful for rapid heating of otherwise slowly prepared cooking items, such as hot butter, fats, and chocolate. Microwave ovens are popular for reheating previously cooked foods and cooking vegetables.

The countertop microwave oven was first introduced in 1967 by the Amana Corporation, which was acquired in 1965 by Raytheon. 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 Named the 'Radarange', it was first sold in 1946. Percy Spencer invented the first microwave oven after World War II from radar technology developed during the war. Microwave ovens heat foods quickly and efficiently because excitation is fairly uniform in the outer 25–38 mm (1–1.5 inches) of a dense (high water content) food item food is more evenly heated throughout (except in thick, dense objects) than generally occurs in other cooking techniques. A microwave oven, often colloquially shortened to microwave, is a kitchen appliance that heats food by bombarding it with electromagnetic radiation in the microwave spectrum causing polarized molecules in the food to rotate and build up thermal energy in a process known as dielectric heating.

Physics investigatory project class 11