Radiation



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

Radiation can be classified as either ionizing or non-ionizing according to whether it ionizes or does not ionize ordinary chemical matter. The word radiation is often colloquially used in reference to ionizing radiation (e.g. x-rays, gamma rays), but the term radiation may correctly also refer to non-ionizing radiation (e.g., radio waves, microwaves, heat or visible light) as well. 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 expands as it passes through space, and as its energy is conserved (in vacuum), the power of all types of radiation follows an inverse-square law in relation to the 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, 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 that do not produce a significant temperature rise. Ultraviolet radiation in some aspects occupies 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 (an example is sunburn). These properties derive from ultraviolet's power to alter chemical bonds, even without having quite enough energy to ionize atoms.

