In physics, radiation is a process in which energetic particles or energetic waves travel through vacuum, or through matter-containing media that are not required for their propagation. Waves of a massive medium itself, such as water waves or sound waves, are usually not considered to be forms of "radiation" in this sense. By contrast, gravitational waves, which are waves of space-time itself, qualify as a type of radiation.

Two energies of radiation are commonly differentiated by the way they interact with normal chemical matter: ionizing and non-ionizing radiation. The word radiation is often colloquially used in reference to ionizing radiation (i.e., radiation having sufficient energy to ionize an atom), but the term radiation may correctly also refer to non-ionizing radiation (e.g., radio waves, heat or visible light). The particles or waves radiate (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 radiation radiates through space and its energy is conserved in vacuum, the power of all types of radiation follows an inverse-square law of power with regard to distance from its source.

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 by ionizing radiation, even at low radiation powers, have the potential to cause DNA damage. By contrast, most non-ionizing radiation is harmful to organisms only in proportion to the thermal energy deposited, and is conventionally considered harmless at low powers which do not produce significant temperature rise. Ultraviolet radiation in some aspects occupies a middle ground, in having some features of both ionizing and non-ionizing radiation. Although nearly all of the ultraviolet spectrum of radiation is non-ionizing, at the same time ultraviolet radiation does far more damage to many molecules in biological systems than is accounted for by heating effects (an example is 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-ionization 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 conventionally 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.

Read more about Radiation:  Ionizing Radiation, Non-ionizing Radiation, Discovery

Other articles related to "radiation":

Synchrotron Radiation
... The electromagnetic radiation emitted when charged particles are accelerated radially is called synchrotron radiation ... It is similar to cyclotron radiation except that synchrotron radiation is generated by the acceleration of ultrarelativistic charged particles through ... Synchrotron radiation may be achieved artificially in synchrotrons or storage rings, or naturally by fast electrons moving through magnetic fields ...
Infrared Window - Kinetics of The Infrared Atmospheric Window
... two radiative components, window and non-window radiation, that are not of the kind that have kinetics suitable for description by the Beer-Lambert law ... The window radiation and the non-window radiation from the land-sea surface are not defined in the terms that are necessary for the application of the Beer-Lambert Law ... the Beer-Lambert Law either to window or non-window radiation considered separately ...
Pedro Albizu Campos - Later Years and Death
... He alleged that he was the subject of human radiation experiments in prison and stated that he could see colored rays bombarding him ... When he wrapped wet towels around his head in order to shield himself from the radiation, the prison guards ridiculed him as El Rey de las Toallas (The King of the Towels) ... to examine Albizu, and test him for signs of radiation ...
Background - Physics and Science
... independence, a condition in theoretical physics Background noise Background radiation, the natural radiation that is always present in a location Cosmic microwave background radiation, a natural constant ...
Uses of Radiation - In Science
... Radiation is used to determine the composition of materials in a process called neutron activation analysis ... The scientists can identify the elements in the sample by studying the emitted radiation ...

Famous quotes containing the word radiation:

    There are no accidents, only nature throwing her weight around. Even the bomb merely releases energy that nature has put there. Nuclear war would be just a spark in the grandeur of space. Nor can radiation “alter” nature: she will absorb it all. After the bomb, nature will pick up the cards we have spilled, shuffle them, and begin her game again.
    Camille Paglia (b. 1947)