Radiation

RADIATION, or radiance, in a general sense, is the process of transferring energy from a body into the surrounding space. Usually, the term radiation is applied to elementary atomic or molecular processes, distinguishing between two types of radiation: corpuscular and light. The transfer of energy in corpuscular radiation (cathode, anode, or canal rays, radioactive alpha and beta rays) is carried out by a stream of material charged particles—electrons or ions. However, radiation is predominantly called light radiation. In this narrow sense, radiation is understood in the present article. The main feature uniting the infinite variety of types of light is the speed of propagation. According to the most accurate measurements by Michelson, the speed of light in a vacuum is equal to 299,796 ± 4 km/sec. All types of light, from radio waves to gamma rays of radium, propagate at this speed in empty space. The second necessary feature of light radiation, distinguishing it from corpuscular radiation, is the absence of free electric charges in the light flux. The third fundamental property of light is its periodicity, manifested in interference and diffraction phenomena. The study of electromagnetic waves has shown that these waves possess all the fundamental properties of light and that, conversely, all types of light radiation can be considered a process of the propagation of electromagnetic waves in space.

Another group of experimental data, mainly from the field of the effects of light, shows, however, that the light flux possesses, besides wave properties, other discontinuous, quantum properties. To explain many phenomena, one has to assume that the energy of light is concentrated in special centers, "light quanta," possessing energy hv and momentum h/λ (here v is the frequency of light waves, i.e., the number of oscillations per second, h is the universal constant equal to 6.55 × 10^-27 erg/sec, c is the speed of light). Light can be absorbed and emitted only in whole quanta. Thus, one has to simultaneously attribute to light both continuous wave properties and a discontinuous quantum character. The task of theoretical synthesis of both properties into a single, consistent image is not yet finished.

Types of light radiation. Interference and diffraction phenomena allow for the measurement of the wavelength λ of light radiation. This length λ is the feature necessary and sufficient for distinguishing one type of light radiation from another. It should be noted that λ changes when light passes from one medium to another, since the speed of light changes in this process. Therefore, it is more rational to characterize light by the ratio v = c/λ, i.e., by the frequency of oscillations, since v is constant in all media. The table shows the values of λ (in a vacuum), v, and the quantum hv (in ergs) for various groups of light radiation, starting from radio waves, which can be obtained as arbitrarily long as desired, to unusually small gamma rays and cosmic radiation. [The units used for λ in the table are as follows: 1μ = 0.001 mm; 1mμ = 0.001 μ; 1Å (angstrom) = 0.1 mμ; 1 X = 0.001 Å.] The division into groups is conventional; the indicated boundaries are in many cases very indefinite. It is evident from the table that light waves of any length exist and have been detected—from infinitely large to practically infinitely small.

Sources of radiation. The subdivision of radiation into groups in the table, as stated, is conventional; however, it has great fundamental and practical significance regarding the sources and effects of radiation. Artificial electromagnetic waves are emitted by entire systems of conductors (vibrators) and in natural conditions are detected mainly only in atmospheric phenomena. The spectrum includes: Infrared (thermal) spectrum 0.3 mm–0.75 μ; Visible spectrum 750 mμ–400 mμ; Ultraviolet spectrum 400 mμ–10 mμ; X-ray spectrum 10 mμ–0.1 Å; Gamma spectrum 1 Å–0.01 Å; Cosmic (penetrating) rays 0.6 X–0.3 X. Infrared rays are predominantly emitted during thermal rotational and vibrational movements of molecules; any heated body serves as an intense source of these rays. Visible and ultraviolet radiation is emitted during changes in the position of electrons in the outer shells of atoms and molecules. Such changes can be excited by collisions of other molecules (high temperature), electron bombardment (in discharge tubes, voltaic arcs), or illumination (fluorescence). To excite ultraviolet rays with a short wavelength, electrons with high speeds are required, obtained, for example, in a discharge spark. X-rays are emitted by atoms during the rearrangement of electrons in the inner shells close to the nucleus; to obtain such changes, electrons with very high speeds are required, obtained in discharge tubes at high voltages. Gamma rays accompany the decomposition of atomic nuclei during radioactive processes, which proceed spontaneously, independently of external conditions. Recently, however, it has been possible to construct discharge tubes operating at enormous voltages, in which X-rays are obtained with the same wavelength as fairly hard gamma rays. The origin of cosmic, extremely hard radiation has not yet been definitively clarified; these rays come to the Earth from outer space and, possibly, are emitted during the formation of new atoms from protons and electrons.

Effects of radiation. Only those rays that are absorbed by a given substance have an effect. Generally speaking, the larger the quantum of absorbed radiation, the more destructive its effect. Long electromagnetic waves upon absorption can only cause heating, which, for example, is sometimes used in diathermy. Thermal rays with a very small quantum can also only increase temperature, exciting rotational and vibrational movements of molecules. Visible and especially ultraviolet rays cause deeper changes in molecules, leading to chemical processes, secondary radiation (fluorescence), or ionization (photoelectric effect). The effect of X-rays and gamma rays is even more destructive. However, in most cases, a large X-ray quantum does not go entirely into the destruction of one molecule; a significant part of it, upon absorption, is converted into the kinetic energy of a secondary electron, which in turn causes changes in other molecules. The X-ray quantum is, as it were, exchanged for smaller ones, thus producing changes in hundreds of molecules. The great therapeutic effect of X-rays and gamma rays is therefore due not so much to the magnitude of their quantum as to its great penetrating power.