http://en.wikipedia.org/wiki/Radiation
From Wikipedia, the free encyclopedia
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium.[1][2] This includes electro-magnetic radiation such as radio waves, visible light, and x-rays, particle radiation such as α, β, and neutron radiation and acoustic radiation such as ultrasound, sound, and seismic waves. Radiation may also refer to the energy, waves, or particles being radiated.
The word arises from the phenomenon of waves radiating (i.e., travel outward in all directions) from a source. This aspect leads to a system of measurements and physical units that are applicable to all types of radiation. Because such radiation expands as it passes through space, and as its energy is conserved (in vacuum), the power of all types of radiation radiating from a point source follows an inverse-square law in relation to the distance from its source. While it is most common that radiation may be emitted radially from a point source, such as a light-bulb filament or a microwave antenna, there are other modes of radiation. Some examples are radiation from a phosphorescent panel (chaotic), a laser beam (coherent), and emitted from a parabolic mirror (parallel), in which cases adherence to the inverse-square law is violated.
EMR is energy transferred by waves of combined electric charge and magnetic monopole, capable of traveling through a vacuum and traveling at the universal speed of light in whatever media it is passing through; the speed is dependent on the media, and is fastest in vacuum. In quantum mechanics these waves have been shown to have particle structure as well as wave structure; these particles are called photons. EMR includes radio and microwave signals, infrared (radiant heat), visible light and ultraviolet, and x-rays and gamma rays. These are differentiated from one another by the frequency of the waves, which directly correlates with the energy carried in each type’s photons. This is the first definition of radiation stated in the opening paragraph.
Notice that the differentiation of radiation into the classes above is somewhat arbitrary. The classes overlap at the meeting points, and the distinctions are strictly man-made, not directly apparent in the physics of the waves under study. There is, for example, no difference between an X-ray and a gamma ray except a relative difference in frequency, and thus energy.
This spectrum of radiant energy can be divided into ionizing and non-ionizing, according to whether it ionizes or does not ionize the atoms in ordinary chemical matter. Ionization is the removing of electrons from atoms, and it may be partial, in which the weaker held outer electrons are removed, grading upwards to removal of all electrons from an atom. The energy required to do this varies with the kinds of atoms and their physical state, such as temperature, chemical binding and so on. Some overlap of ionizing and non-ionizing radiation exists in the domain of ultraviolet where materials experience first simple thermal heating in the infrared and visible light, then excitation of electrons in “softer” UV, and then partial-to-total ionization as the energy increases with frequency. The second definition of radiation in the opening paragraph is used in reference to ionizing radiation in hard UV, x-rays, and gamma rays.
Both ionizing and non-ionizing radiation can be harmful to organisms and can result in changes to the natural environment. In general, however, ionizing radiation is far more harmful to living organisms per unit of energy deposited than non-ionizing radiation, since the ions that are produced, even at low radiation powers, leave behind atoms which, due to charge imbalance, are eager to combine in semi-random ways with other atoms in the environment; these are called free radicals. Such random chemical action in a cell may result in anything from harmless reactions, to degradation of important structures in the cell, to killing it outright or triggering suicide (apoptosis), or modifying the DNA in harmful, but yet temporarily viable ways. By contrast, most non-ionizing radiation is harmful to organisms only in proportion to the thermal energy deposited (a prime example is microwaves generated in a microwave oven), and is conventionally considered harmless at low powers that do not produce a significant temperature rise. Ultraviolet radiation in some aspects occupies the overlap in a middle ground, as it has some features of both ionizing and non-ionizing radiation. Although nearly all of the ultraviolet spectrum that penetrates the Earth’s atmosphere is non-ionizing, this radiation does far more damage to many molecules in biological systems than can be accounted for by heating effects, such as sunburn). These properties derive from ultraviolet’s power to alter chemical bonds, even without having quite enough energy to ionize atoms.
The question of harm to biological systems due to low-power ionizing and non-ionizing radiation is not settled. Controversy continues about possible non-heating effects of low-power non-ionizing radiation, such as non-heating microwave and radio wave exposure. Non-ionizing radiation is usually considered to have a safe lower limit, especially as thermal radiation is unavoidable and ubiquitous. By contrast, ionizing radiation is currently conservatively considered to have no completely safe lower limit, although at some energy levels, new exposures do not add appreciably to background radiation. The evidence that small amounts of some types of ionizing radiation might confer a net health benefit in some situations is called radiation hormesis.
