Microwave ovens use high-energy electromagnetic waves (or microwaves) to heat foods by energizing the food molecules, causing them to reverse polarity millions of times per second. The resulting friction generated by the molecules heats the food. No external source of heat is used in microwave cooking.
The key component of a microwave oven, and easily the most expensive part is a magnetron tube. This element produces electromagnetic waves called microwaves that are reflected by metal and absorbed by moisture-containing substances such as food.
Unlike ionizing radiation (such as X-rays or gamma rays) that can alter molecular structure, non-ionizing microwaves have no permanent effect on food molecules.
Unless specific instructions are provided, metal should not be placed in microwave ovens. Metal prevents the microwaves from reaching foods, and may cause arcing between the metal and oven walls, reducing the life or damaging the expensive magnetron tube.
Microwaves occur between radio waves and visible light in the electromagnetic spectrum. In addition to use in residential and commercial food ovens, microwaves are used in communication and radar systems, and have many other commercial applications.
A microwave oven has a unit which produces radio waves with a frequency of around 2.5 gigahertz. The radio waves are of a frequency which is most absorbed by food. When the microwaves fall on the food, the molecules of food absorb the energy in the radiation and this causes them to start vibrating. Due to the vibration of the molecules, the food gets hot and is cooked.
In microwave cooking the maximum absorption is just under the outer layer of the food as the microwaves lose energy when they strike the food. From here there is an inward and outward transfer of heat which ultimately cooks the food. Unlike conventional cooking which requires water or air to transfer heat to the food, microwaves directly heat up the food molecules and no medium of transfer is required.
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