Microwaves have wavelengths more than that of infrared waves, but less than that of radio waves. Although they were first produced and studied in 1886 by Hertz, their practical application was not before the invention of suitable generators. The name "Microwave" is just a misnomer. The rays are not at all micro.
Microwave transmitters and receivers are parabolic dish antennas, which produce beams whose spreading angle depends upon their diameter, and can be directed like a searchlight.
A RADAR (Radio Detection and Ranging) is one of the most commonly used transmitters for guiding airplanes and vessels and for detecting speeding motorists. Radar beams consist of short pulses of microwaves. One can determine the distance of an airplane or ship by measuring the time it takes such a pulse to travel to the object and back to the radar dish antenna after reflection. Besides, by using the change in frequency of the reflected wave pulse caused by the Doppler effect, one can also measure the speed of objects.
Because a radar senses electromagnetic energy that was actively transmitted, it is considered an active remote sensing system. Passive remote sensing refers to the sensing of electromagnetic energy which does not originate from the satellite or sensor. Today, various types of microwave generators and amplifiers have been developed, like the klystron and the magnetron, two vacuum tube devices, which continue to be used on a wide scale, especially for higher-power applications.
Klystrons are mainly used as amplifiers in radio relay systems and for dielectric heating, whereas magnetrons have been employed for radar systems. Solid-state technology has yielded several devices capable of producing, amplifying, detecting, and controlling microwaves.
Microwaves are the principal carriers of high-speed telegraphic data transmissions between Earth stations and with satellites and space probes. A system of synchronous satellites about 36,000 km above the Earth is used for international broadband telegraphy of all kinds of tele-communications.
Microwaves can penetrate clouds of smoke, but are scattered by water droplets, and hence can be used for mapping meteorologic disturbances and in weather forecasting.
Maser (Microwave Amplification by Stimulated Emission of Radiation) has proved very useful in areas such as radio astronomy, microwave radiometry, and long-distance communications.
Natural masers in some interstellar clouds have been discovered by astronomers. Observations of radio radiation from interstellar hydrogen (H2) and certain other molecules has indicated amplification by the maser process.
A microwave cosmic background radiation has also been detected in the space and is considered by many scientists as the remnant of the big-bang.
A very common device, microwave oven is a heating device in which the waves generated are tuned to the frequency that can be absorbed by the water and fat in the food, but not by the dish, which is relativley dry, and is thus not heated up. When a microwave photon strikes a water molecule, the latter, being polar, rotates to align itself with the incoming electromagnetic field. This movement increases the temperature of the substance in which that particular water molecule is present.
Microwaves ovens play an increasingly wide role in heating and cooking food quickly, and are being widely used today.
The heating effect of microwaves destroys living tissue when the temperature of the tissue exceeds 43º C (109º F). Accordingly, prolonged exposure to intense microwaves to body surface can be harmful. In extreme cases, it can lead to cataracts, and can have adverse effects on the electrochemical balance of the brain, if the frequency equals the brain wave frequencies.