RADIATION. – By the generic term "radiation" (r.) one may indicate either a flux of particles or corpuscles or a flux of electromagnetic waves: that is, there is corpuscular radiation and electromagnetic radiation. Natural radioactive substances emit α and β corpuscular radiation and γ electromagnetic waves. All incandescent bodies emit electromagnetic radiation; to this same type of radiation also belong Hertzian waves and X-rays.
I. CORPUSCULAR RADIATION. – The energy E of corpuscles depends on their mass m and their velocity v (E = ½ m·v²). The velocity v of corpuscles cannot exceed the velocity c of light; when v is only slightly less than c, the energy is represented by a formula different from the one indicated. Corpuscular radiation may consist of electrons emitted by radioactive substances or accelerated by suitable potential differences: their energy is measured by the charge e multiplied by the accelerating potential V, i.e., one eV (electron-volt). Ions have a charge equal to or a multiple of that of the electron (e.g., protons and deuterons have charge e; α particles, 2e, etc.); corpuscular radiation consisting of accelerated ions therefore has twice the energy of electrons accelerated with the same potential difference. The α particles from natural radioactive substances have energies of several million eV; the corpuscles that are part of cosmic radiation reach energies exceeding billions of eV; with modern accelerating machines (cyclotrons, betatrons, bevatrons, etc.), corpuscles can be accelerated to energies of millions and billions of eV.
II. ELECTROMAGNETIC WAVES. – All electromagnetic waves have in common the propagation velocity c, which in a vacuum is about 300,000 km/sec. They differ, however, in wavelength λ and frequency ν. Since λν = c, greater wavelengths correspond to lower frequencies and vice versa. Classified in increasing order of λ, electromagnetic waves include: a) electromagnetic waves of cosmic rays; b) γ radiation emitted by radioactive substances; c) X-rays produced in special tubes by the impact of accelerated electrons against matter; d) ultraviolet, visible, and infrared radiation emitted by all incandescent bodies; the electromagnetic radiation perceptible to the sense of sight are the most important and the most anciently known, as they are used for this primary function, but from a physical point of view they do not differ, apart from λ and ν, from the other electromagnetic radiation; e) Hertzian waves, first obtained by Hertz and now widely used in all kinds of radio communications.
One speaks of the "spectrum" of a radiation to indicate the range of λ that compose II. A solid or liquid incandescent body emits a continuous spectrum, i.e., the radiation it emits includes all possible λ. The visible spectrum is the set of radiation perceptible to the human eye. It covers all λ from 0.4 to 0.8 microns (1 micron = 1/1000 mm). Visible radiation of different λ is perceived by the eye as different color sensations. Just as there are infinite λ between 0.4 and 0.8 microns, so there are infinite colors; these have been classified by Newton into 7 conventional regions (red, orange, yellow, green, blue, indigo, violet). Newton was also able to demonstrate that solar white light is composed of radiation of all colors by decomposing the same light through a prism.
Electromagnetic radiation constitutes a form of energy (radiant energy) that propagates with velocity c. When this energy is controlled or measured, it appears concentrated in small energy packets called "quanta" or "quanta," whose value depends on ν and precisely E = h·ν, where h is a universal constant (Planck's constant) equal to 6.6·10⁻²⁷ erg·sec. The energy of quanta therefore depends on the ν of the radiation to which they belong. This energy is usually measured in eV; quanta of visible light have energies on the order of 1 eV, while the quanta of X-rays range from 100,000 eV to 10 million eV (i.e., up to 10 MeV).
In the case of solid or liquid incandescent bodies, the spectral distribution of the emitted radiant energy depends mainly on the temperature of the body. This spectral distribution is shown in the figure for different absolute temperatures T of the emitting body (it may be considered that 3000 K is the T of the filament of an electric lamp, 6000 K the T of the surface of the sun, 600,000 K that of the incandescent globe of an atomic bomb). It is to be noted that the maximum of the three curves falls respectively in the infrared, visible, and X-ray regions of the spectrum. More precisely, Wien's law states that the maximum wavelength (λm) is inversely proportional to T, i.e., λm = 2900/T (2900 micron degrees = Wien's constant). For example, for the sun the radiant energy shows a maximum at λ = 2900:6000 = 0.48 microns, which corresponds to yellow-green light. The total energy emitted (for all wavelengths) is proportional to the fourth power of T (i.e., to T⁴). One cm² of the atomic bomb, assumed to be at a temperature one hundred times greater than the sun, emits 100⁴ = 100 million times more energy than 1 cm² of the sun. Incandescent gaseous bodies emit a discontinuous spectrum, i.e., radiation belonging to certain λ; for example, sodium vapor emits in the visible spectrum only yellow radiation of λ = 0.589 microns, hydrogen emits 4 visible radiations of red, blue, indigo, and violet colors (with λ = 0.656; 0.486; 0.434; 0.410 microns).
III. DETECTION OF RADIATION. – Radiation is detected on the basis of the effects it produces. The eye is the natural detector of the radiation that constitutes the visible spectrum. In general, if a body absorbs any radiation, it transforms its energy into heat, i.e., it can detect it through its heating (thermocouple). In some cases, the incident radiation causes the emission of electrons (photoelectric effect), used in the photoelectric cell now widely employed in sound films, television, etc. Finally, electromagnetic and corpuscular radiation can produce fluorescence, phosphorescence, ionization, blackening of photographic emulsions, and other physical-chemical effects.
BIBL.:
- Planck, *Vorlesungen über die Theorie der Wärmestrahlung* (Leipzig 1921);
- Sommerfeld, *Atombau und Spektrallinien* (Braunschweig 1939);
- Fermi, *Fisica* (Torino 1945);
- Rossi, *I raggi cosmici* (Bologna 1941).