In physics, radiation is the energy emitted or transmitted in the form of waves or particles through space or material medium, including:

According to the energy of radiation particles, radiation is usually divided into ionization or non ionization. Ionizing radiation carries more than 10 EV, enough to ionize atoms and molecules and break chemical bonds. Because the harm to living organisms is very different, it is an important difference. The common source of ionizing radiation is the radioactive material emitted or radiated, which is composed of helium nucleus, electron or positron and photon respectively. Other sources include X-rays from medical radiography and mesons, mesons, positrons, neutrons, and other particles that make up the secondary cosmic rays that interact with the earth's atmosphere.

The higher energy ranges of gamma, X-ray and ultraviolet constitute the ionization part of the electromagnetic spectrum. The term "ionization" refers to the splitting of one or more electrons from atoms, which requires relatively high energy provided by these electromagnetic waves. Looking down, the non ionized lower energy of the lower UV spectrum does not ionize the atom, but it can destroy the intermolecular bond that forms the molecule, thus destroying the molecule rather than the atom. A good example is the sunburn caused by long wavelength ultraviolet rays. Waves with longer wavelengths than UV in visible, infrared, and microwave frequencies do not destroy the bond, but cause vibrations in the bond that are perceived as heat. It is generally believed that the wavelength of radio wave long or below is harmless to biological system. These are not clear descriptions of energy; the effects of specific frequencies overlap.

The phenomenon of a wave that radiates from its source (that is, travels outward in all directions). This aspect leads to systems that are suitable for all types of radiation measurements and physical units. Since such radiation expands as it passes through space and retains energy (in a vacuum), all types of radiation intensity from a point source follow the inverse square law related to the distance from its source. Like any ideal law, inverse square law approximates the measured radiation intensity to the extent that the light source approximates the geometric point.