RADIATIONS. - Under the generic name of r. one may designate both the flow of particles or corpuscles and that of electromagnetic waves: that is, there is a corpuscular r. and an electromagnetic r. Natural radioactive substances emit α and β corpuscular r. and γ electromagnetic waves. All incandescent bodies emit electromagnetic r.; to the same type of r. belong also Hertzian waves and X-rays.
I. CORPUSCLES
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 slightly less than c, the energy is represented by a formula different from the one indicated. Corpuscular r. can be constituted by electrons emitted by radioactive substances or else accelerated by suitable potential differences: their energy is measured by the charge multiplied by the accelerating potential V, that is, in eV (electron volts). Ions have a charge equal to or a multiple of that of the electron (for example, protons and deuterons have charge e; α particles, 2 e, etc.); corpuscular r. constituted by accelerated α particles therefore have an energy double that of electrons accelerated with the same potential difference. The α particles of natural radioactive substances have energies of a few million eV; the corpuscles that form part of cosmic r. reach energies above billions of eV; with modern accelerating machines (cyclotrons, betatrons, bevatrons, etc.) corpuscles can be accelerated up to energies of millions and billions of eV.
II. ELECTROMAGNETIC WAVES
All electromagnetic waves have in common the speed of propagation c, which in a vacuum is about 300,000 km/sec. They differ, however, in wavelength λ and in frequency ν. Since λν = c, the longer wavelengths correspond to the lower frequencies and vice versa. Classified in increasing order of λ, electromagnetic waves comprise: a) electromagnetic waves of cosmic rays; b) γ rays emitted by radioactive substances; c) X rays produced in special tubes by the impact of accelerated electrons against matter; d) ultraviolet, visible, and infrared rays emitted by all incandescent bodies; the electromagnetic rays perceptible by the sense of sight are the most important and the most anciently known, inasmuch as they are used for this primary function, but from the physical point of view they do not differ, apart from their λ and ν, from the other electromagnetic rays; e) Hertzian waves, first obtained by Hertz and today widely used in radio communications of every kind.
The "spectrum" of a radiation is commonly taken to mean the range of the λ that compose it. An incandescent solid or liquid body emits a continuous spectrum, that is, the rays emitted by it comprise all possible λ. The visible spectrum is the set of rays perceptible by the human eye. It embraces all wavelengths from 0.4 to 0.8 micron (1 micron = 1/1000 of a mm.). Visible rays of different λ are perceived by the eye with different sensations of color. Just as between 0.4 and 0.8 micron there exist infinite wavelengths, so there exist infinite colors; these were classified by Newton into 7 conventional regions (red, orange, yellow, green, blue, indigo, violet). The same Newton was able to demonstrate that white sunlight is made up of rays of all colors, by decomposing that same light by means of a prism.
Electromagnetic rays constitute a form of energy (radiant energy) that propagates with the speed c. When this energy is controlled or measured, it appears concentrated in many granules of energy called "quanta," whose value depends on the ν and precisely E = h.v, where h is a universal constant (Planck's constant) equal to 6.6.10⁻²⁷ erg. sec. The energy of the quanta therefore depends on the ν of the radiation to which they belong. This energy is customarily measured in eV; the quanta of visible light have energy of the order of 1 eV, while the quanta of X rays reach from 100,000 eV to 10 million eV (that is, to 10 MeV).
In the case of incandescent solid or liquid bodies, the spectral distribution of the emitted radiant energy depends above all on the temperature of the body. This spectral distribution is shown in the figure for the different absolute temperatures T of the emitting body (one may take 5000° K to be the T of the filament of a light bulb, 6000° K the T of the surface of the sun, 600,000° K that of the incandescent globe of an atomic bomb). Note that the maximum of the three curves falls respectively in the infrared, visible, and X regions of the spectrum. More precisely, Wien's law establishes that the "maximum λ" (λm) is inversely proportional to the T, that is, λm = 2900/T (2900 micron degrees = Wien's constant). For example, for the sun the radiant energy presents a maximum at λ = 2900: 6000 = 0.48 micron, which corresponds to yellow-green light. The total energy emitted (for all wavelengths) proves to be proportional to the 4th power of the T (that is, to T⁴). One cm² of the atomic bomb, assumed to be at a T a hundred times greater than the sun's, emits an energy 100⁴ = 100 million times greater than 1 cm² of sun. Incandescent gaseous bodies emit a discontinuous spectrum, that is, rays belonging to certain λ; for example, sodium vapors emit in the visible spectrum only yellow rays of λ = 0.589 micron; hydrogen emits 4 visible rays of red, blue, indigo, and violet color (with λ = 0.656; 0.486; 0.434; 0.410 micron).
III. DETECTION OF RADIATION
Radiation is detected on the basis of the effects it produces. The eye is the natural detector of the electromagnetic radiation that constitutes the visible spectrum. In general, if a body absorbs any radiation it transforms its energy into heat, that is, it can detect the radiation itself by means of its heating (thermoelectric couple). In some cases the incident radiation causes a release of electrons (photoelectric effect), used in the photoelectric cell, today widely employed in sound film, television, etc. Electromagnetic and corpuscular radiations can, finally, produce fluorescence, phosphorescence, ionization, blackening of photographic emulsions and other physicochemical effects.
Tito Franzini