Radiation
In physics,
radiation is a process in which electromagnetic waves travel through a vacuum or through matter-containing media; the existence of a medium to propagate the waves is not required. A different but related definition says
radiation is a subset of these electromagnetic waves combined with a class of energetic subatomic particles with very high kinetic energies; these are called
ionizing radiation, and the particles are termed particle radiation. Other sorts of waves, such as acoustic, seismic, hydraulic and so on are not usually considered to be forms of "radiation" in either sense. We will consider the first definition, and return to the second later. The word arises from the phenomenon of waves
radiating 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, 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.