Editing Electromagnetic spectrum (section)

=== Radio waves ===
{{Main|Radio waves}}
{{Further|Radio frequency|Radio spectrum}}

[[Radio]] waves are emitted and received by [[antenna (radio)|antennas]], which consist of conductors such as metal rod [[resonator]]s. In artificial generation of radio waves, an electronic device called a [[transmitter]] generates an alternating [[electric current]] which is applied to an antenna. The oscillating electrons in the antenna generate oscillating [[electric field|electric]] and [[magnetic field]]s that radiate away from the antenna as radio waves. In reception of radio waves, the oscillating electric and magnetic fields of a radio wave couple to the electrons in an antenna, pushing them back and forth, creating oscillating currents which are applied to a [[radio receiver]]. Earth's atmosphere is mainly transparent to radio waves, except for layers of charged particles in the [[ionosphere]] which can reflect certain frequencies.

Radio waves are extremely widely used to transmit information across distances in [[radio communication]] systems such as [[radio broadcasting]], [[television]], [[two way radio]]s, [[mobile phone]]s, [[communication satellite]]s, and [[wireless networking]]. In a radio communication system, a radio frequency current is [[modulation|modulated]] with an information-bearing [[signal]] in a transmitter by varying either the amplitude, frequency or phase, and applied to an antenna. The radio waves carry the information across space to a receiver, where they are received by an antenna and the information extracted by [[demodulation]] in the receiver.  Radio waves are also used for navigation in systems like [[Global Positioning System]] (GPS) and [[navigational beacon]]s, and locating distant objects in [[radiolocation]] and [[radar]]. They are also used for [[remote control]], and for industrial heating.

The use of the [[radio spectrum]] is strictly regulated by governments, coordinated by the [[International Telecommunication Union]] (ITU) which [[frequency allocation|allocates frequencies]] to different users for different uses.

==== Microwaves ====
{{Main|Microwaves}}
[[File:Atmospheric electromagnetic opacity.svg|thumb|right|upright=1.6|Plot of Earth's atmospheric opacity to various wavelengths of electromagnetic radiation. This is the surface-to-space opacity, the atmosphere is transparent to [[longwave]] radio transmissions within the [[troposphere]] but opaque to space due to the [[ionosphere]].]]
[[File:2013 Atmospheric absorption of electromagnetic waves.svg|thumb|upright=1.6|Plot of atmospheric opacity for terrestrial to terrestrial transmission showing the molecules responsible for some of the resonances]]

[[Microwave]]s are radio waves of short [[wavelength]], from about 10 centimeters to one millimeter, in the [[super-high frequency|SHF]] and [[extremely high frequency|EHF]] frequency bands. Microwave energy is produced with [[klystron]] and [[magnetron]] tubes, and with [[Solid-state electronics|solid state]] devices such as [[Gunn diode|Gunn]] and [[IMPATT diode]]s. Although they are emitted and absorbed by short antennas, they are also absorbed by [[polar molecule]]s, coupling to vibrational and rotational modes, resulting in bulk heating. Unlike higher frequency waves such as [[infrared]] and [[visible light]] which are absorbed mainly at surfaces, microwaves can penetrate into materials and deposit their energy below the surface. This effect is used to heat food in [[microwave oven]]s, and for industrial heating and medical [[diathermy]]. Microwaves are the main wavelengths used in [[radar]], and are used for [[satellite communication]], and [[wireless networking]] technologies such as [[Wi-Fi]]. The copper cables ([[transmission line]]s) which are used to carry lower-frequency radio waves to antennas have excessive power losses at microwave frequencies, and metal pipes called [[waveguide]]s are used to carry them. Although at the low end of the band the atmosphere is mainly transparent, at the upper end of the band absorption of microwaves by atmospheric gases limits practical propagation distances to a few kilometers.

[[Terahertz radiation]] or sub-millimeter radiation is a region of the spectrum from about 100&nbsp;GHz to 30 terahertz (THz) between microwaves and far infrared which can be regarded as belonging to either band. Until recently, the range was rarely studied and few sources existed for microwave energy in the so-called ''[[terahertz gap]]'', but applications such as imaging and communications are now appearing. Scientists are also looking to apply terahertz technology in the armed forces, where high-frequency waves might be directed at enemy troops to incapacitate their electronic equipment.<ref>{{cite news|title=Advanced weapon systems using lethal Short-pulse terahertz radiation from high-intensity-laser-produced plasmas |date=March 6, 2005 |url=http://www.indiadaily.com/editorial/1803.asp |work=India Daily |access-date=2010-09-27 |url-status=dead |archive-url=https://web.archive.org/web/20100106223741/http://indiadaily.com/editorial/1803.asp |archive-date=6 January 2010 }}</ref> Terahertz radiation is strongly absorbed by atmospheric gases, making this frequency range useless for long-distance communication.