Energy in the form of electromagnetic waves (also called radiant energy, or light). It is emitted from matter in the form of photons (quanta), each with an associated electromagnetic wave having frequency (ν) and wavelength (+ll). The various forms of radiant energy are characterized by their wavelength, and together they compose the electromagnetic spectrum, the components of which are as follows: (1) cosmic rays (highest energy, shortest wavelength), (2) γ-rays from radioactive disintegration of atomic nuclei, (3) X-rays, (4) UV rays, (5) visible light rays, (6) infrared, (7) microwave, and (8) radio (Hertzian) and electric rays. All these are identical in every way except wavelength, those having the shortest wavelength being the most penetrating. They are not electrically charged and have no mass, their velocity of propagation is the same, all display the properties characteristic of light and have a dual nature (wave-like and corpuscular). In a looser sense the term radiation also includes energy emitted in the form of particles that possess mass and may or may not be electrically charged, (i.e., α [positive] and β [negative]) and also neutrons. Beams of such particles may be considered as “rays.” The charged particles may all be accelerated and the high energy imparted to “beams” in particle accelerators such as cyclotrons, betatrons, synchrotrons, and linear accelerators.Radiation is used in medicine in the form of X-rays and radioactive isotopes; it is used in industry in many ways, e.g., as vitamin activator, sterilizing agent, and polymerization initiator; it is also the basis of all types of spectroscopic analysis.
