microwave
A microwave oven, or a microwave, is a kitchen appliance that cooks or heats food by dielectric heating. [...] Additional kinds of heat sources can be added to microwave packaging, or into combination microwave ovens, to add these additional effects. [...] This excitation is fairly uniform, leading to food being adequately heated throughout (except in thick objects), a feature not seen in any other heating technique. [1]
Microwaves are electromagnetic waves with wavelengths ranging from as long as one meter down to as short as one millimeter, or equivalently, with frequencies between 300MHz (0.3 GHz) and 300 GHz. [...] In all cases, microwave includes the entire SHF band (3 to 30GHz, or 10 to 1cm) at minimum, with RF engineering often putting the lower boundary at 1GHz (30cm), and the upper around 100GHz (3mm). [2]
The boundaries between far infrared light, terahertz radiation, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used variously between different fields of study. [3]
This makes them unsuitable for cooking certain foods, or to achieve certain effects. [1]
Microwave ovens can play an important role at mealtime, but special care must be taken when cooking or reheating meat, poultry, fish, and eggs to make sure they are prepared safely. [...] Microwave ovens can cook unevenly and leave “cold spots” where harmful bacteria can survive. [4]
Basic microwave ovens heat food quickly and efficiently, but do not brown or bake food in the way conventional ovens do. [...] A microwave oven works by passing non-ionizing microwave radiation, usually at a frequency of 2.45 gigahertz (a wavelength of 12.24 centimetres (4.82 in)), through the food. [1]
As a consequence, practical microwave technique tends to move away from the discrete resistors, capacitors, and inductors used with lower frequency radio waves. [2]
Water, fat, and other substances in the food absorb energy from the microwaves in a process called dielectric heating. [1]
For this reason, it is important to use a food thermometer and test food in several places to be sure it has reached the recommended safe temperature to destroy bacteria and other pathogens that could cause foodborne illness. [...] At that frequency, power is readily absorbed by water, fats and sugars, resulting in very fast vibration and high temperatures that cook the food. [4]
Sources:
[1] Microwave oven - Wikipedia, the free encyclopedia
[2] Microwave: Definition from Answers.com
[3] Microwave - Wikipedia, the free encyclopedia
[4] Microwave Ovens and Food Safety