Radiation

Radiation has a variety of different meanings; some are unfortunately very similar.

• Radiation can mean the energy emitted by a source into the surrounding environment. This can take a number of forms:
  • Ionizing radiation is a stream of particles (photons or other particles) with sufficient energy to cause ionization of atoms or molecules.
  • Electromagnetic radiation is a stream of photons, of a variety of different energies. The nature and effects depend strongly on the energy per photon as well as the total energy in the radiation. With sufficient energy per photon, electromagnetic radiation can be ionizing radiation.
  • Gravitational radiation is a predicted consequence of general relativity, however, it has not been observed. In quantum models of gravity, it would be made up of particles called gravitons.
  • Particle radiation is any kind of radiation in which the individual elements behave like particles; this includes both ionizing radiation and neutron radiation and possibly also radiation consisting of neutrinos, but it is not normally used to mean low-energy electromagnetic radiation or gravitational radiation.
• Radiation can be a process by which a source emits energy. There are many such processes. A few are:
  • Thermal radiation is the process by which a hot object emits electromagnetic radiation; it is one of the three methods of heat flow. If the emitting object is "black", that is, shows no spectral preference for one wavelength over another, the resulting radiation is called blackbody radiation and has a characteristic spectrum from which the temperature can be calculated.
  • Cerenkov radiation is the emission of radiation by a particle moving through an insulating medium faster than the speed of light in that medium.
  • Synchrotron radiation is the emission of radiation by an charged particle undergoing acceleration. This happens, for example, if the particle moves in a circle, as in a synchrotron.
• Adaptive radiation is a process in evolutionary biology by which one species becomes many in order to adapt to specific ecological niches. (This usage has little to do with any of the previous usages).

Radiation is sometimes also used, inaccurately, to refer to radioactive contamination, the release of radioactive isotopes into the environment. These isotopes then release ionizing radiation, which can be especially severe if the isotopes have been taken up by plants, animals or humans, since the isotopes then release their ionizing radiation from inside the organism.