Radiation from different wavelengths in the electromagnetic spectrum has very different characteristics and uses.
Radio waves have wavelengths ranging from one millimeter to hundreds of meters and frequencies of about 3 Hz to 300 GHz. They are commonly used by people to transmit data for television, mobile phones, wireless internet connections, and many other applications. The technology to encode radio waves with data is complex but involves changing the amplitude and frequency and phase relations of waves within a specific frequency band.
Microwave radiation has wavelengths ranging from one millimeter to one meter and frequencies between 0.3 GHz and 300 GHz. It includes super high frequency (SHF) and extremely high frequency classes. Microwaves are absorbed by molecules with dipolar covalent bonds, a property that is used to heat material uniformly and rapidly in microwave ovens. Microwave radiation is also used for some communication applications.
Terahertz radiation has wavelengths between the far infrared and microwaves and frequencies between 300 GHz and 3 terahertz. Radiation in this region can be used for imaging and communications and in electronic warfare to disable electronic equipment.
Infrared radiation has wavelengths between visible light and terahertz radiation and frequencies of 300 GHz (1 mm) to 400 THz (750 nm). Far-infrared radiation (300 GHz to 30 THz) is absorbed by the rotation of many gas molecules, by the molecular motions in liquids, and by phonons (a quantized mode of vibration of a crystal lattice) in solid phases. Most of the far-infrared radiation that enters the Earth's atmosphere is absorbed except for a few wavelength ranges (called windows) where some energy can penetrate. Mid-infrared radiation has frequencies from 30-120 THz and includes thermal radiation from blackbodies (i.e., bodies that absorb all energy at all wavelengths when they are cold). Near-infrared radiation is similar to visible light and has frequencies from 120 to 400 THz.
Higher frequency radiation (between 400 and 790 THz) with wavelengths between 400 and 700
nanometers is detectable by the human eye. Known as visible light, this form of radiation is also the range of most of the radiation emitted from the sun and stars. When objects reflect or emit light in the visible range, the human eye and brain are able to process data from these wavelengths into an optical image of the object. The details of how the human brain perceives radiation from these wavelengths and processes it into an image are not completely understood. Molecular biologists, neuroscientists, psychologists, and biophysicists are actively studying these processes.
Ultraviolet radiation has wavelengths shorter than visible light and longer than X-rays, falling between 400 and 10 nm, and has energies between 3 and 124 electron volts. Ultraviolet radiation is emitted by the sun and is a highly energetic ionizing radiation that can induce chemical reactions, may cause some substances to glow or fluoresce, and can cause sunburn on human skin. The ultraviolet radiation from the sun is poisonous to most living organisms but is absorbed by the atmospheric ozone layer, preventing significant damage to life on Earth. If the ozone layer is depleted, ultraviolet radiation will cause significant damage to life on the surface of the Earth. During the early history of the Earth, no ozone layer existed, so the surface was constantly drenched in ultraviolet radiation. This may have prevented life from inhabiting the surface until a few billion years after the formation of the planet, when the ozone layer developed.
X-rays have wavelengths from 10 to 0.01 nanometers with frequencies between 30 petahertz and 30 exahertz (30 x 1015 Hz to 30 x 1018 Hz) and energies between 120 eV to 120 keV. X-rays can see through some objects (like flesh) but not others (like bones) and can be used to produce images for diagnostic radiography and crystallography. In the cosmos X-rays are emitted by neutron stars, some nebulae, and the accretion disks around black holes.
Gamma rays are the most energetic photons; they have no lower limit to their wavelength. Their frequency is greater than 1019 Hz, their energies are more than 100 keV, and their wavelengths are less than 10 picometers. Gamma rays are highly energetic and ionizing so they can cause serious damage to human tissue and represent a serious health hazard.
See also Archean; astronomy; cosmic microwave background radiation; electromagnetic spectrum; energy in the Earth system; life's origins and early evolution; ozone hole; pulsar; quasar; radioactive decay; radio galaxies; remote sensing.
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