Microwaves are electromagnetic waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz (0.3 GHz) and 300 GHz.They appear between radio waves and infrared on the electromagnetic spectrum.
Although Microwaves are produced freely in the solar system it is only in the last 70 years that their potential has been exploited. The
development of the Cavity Magnetron during the Second World War provided a reliable means of producing microwaves in a narrow
frequency band. This device has allowed microwaves to be extensively used and become part of everyday life. Radar, GPS, Mobile Phones and Satellite Communications are just a few of the common applications.
Probably the most widely recognised effect of Microwaves is their heating effect and their use in Microwave ovens. The method by which Microwaves heat materials is different to conventional thermal heating systems. When microwaves pass through certain compounds they absorb energy from the microwaves in a process called dielectric heating.
Many molecules (such as those of water) are electric dipoles, meaning that they have a positive charge at one end and a negative charge at the other. They rotate as they try to align themselves with the alternating electric field of the microwaves. This molecular movement represents heat which is then dispersed as the rotating molecules hit other molecules and put them into motion.
Microwave heating is more efficient on certain molecules than others depending on the presence and size of the molecular dipole moment. This can be measured and is known as the dielectric constant. It is also more efficient in liquid water than in ice where the molecules are not free to rotate.
Depending on the constituents of the material the depth of initial heat deposition may be many centimeters or more in a microwave system. This is in contrast to convection heating, which deposit heat thinly at the material surface and rely on conduction to take the heat from the surface to the core. Generally the penetration depth of microwaves is dependent on the composition of the material and the microwave frequency. The lower the microwave frequencies (longer wavelengths) the better the penetrating power.
Certain molecules are transparent to microwave energy. Quartz for instance absorbs very little energy so do certain elements like sulphur. Others like silver and aluminum reflect any incident microwaves.