SCALE OF FREQUENCIES AND WAVES OF ELECTROMAGNETIC OSCILLATIONS;

CLASSIFICATION AND TERMS*

§ 1. Electromagnetic spectrum.
§ 2. Scale of electromagnetic waves.
§ 3. Justification of the need for a standard.
§ 4. Draft standard.
§ 5. Explanations of the draft.

§ 1. The Complete Electromagnetic Spectrum

The complete electromagnetic spectrum includes all electromagnetic waves obtained and observed on the earth, beginning with the longest, of indefinitely great wavelength, down to the shortest, with wavelengths of the order of \(10^{-11}\) cm. All electromagnetic waves are identical in their nature: they obey the same laws of reflection, refraction, interference, diffraction, and polarization, and have the same velocity of propagation in the ether, equal to \(3 \cdot 10^{-10}\) cm/sec; they differ from one another only in wavelength.

The electromagnetic spectrum is the name given to a continuous series of waves arranged in consecutive order according to their length, by analogy with the light spectrum in Newton’s classical experiment. Thus the complete electromagnetic spectrum is the totality of all electromagnetic waves obtained and observed on the earth, arranged in consecutive order according to wavelength. At the present time the complete electromagnetic spectrum is continuous; the last gap in it was filled in 1923. The filling of this gap is of great fundamental importance as experimental confirmation of the unity of the nature of Hertzian and light waves. Owing to the absence of a single method for exciting oscillations for all wavelengths, the complete electromagnetic spectrum has, as it were, a “mosaic” structure—its structural elements are separate regions characterized by different methods of exciting waves; these regions not only fill the entire spectrum completely, but almost always overlap one another at their ends. In the latter case we are dealing with waves of one and the same length obtained by different methods; such overlaps of adjacent regions of the spectrum occur, for example, between X-rays and ultraviolet rays, infrared and transitional (ultra-Hertzian) waves, and in other parts of the spectrum.

§ 2. Scale of Frequencies and Waves of Electromagnetic Oscillations

The complete electromagnetic spectrum is represented graphically on a logarithmic scale of lengths (scala—flight of stairs), in order to make it possible to place the wavelengths of the entire electromagnetic spectrum on a sheet

* In presenting the draft of a mandatory OST with an explanatory note for broad discussion, we ask all interested persons to send their considerations on this draft to the All-Union Committee for Standardization under the STO at the address: Moscow 12, Razina St. 12, VKS under the STO, NTO Sector.

SCALE OF FREQUENCIES AND WAVES OF ELECTROMAGNETIC OSCILLATIONS

paper. For this purpose, equal segments corresponding to the logarithm of a definite number, for example \(\lg 2\) or \(\lg 10\), are laid off on a straight line; in the first case the scale will be represented in octaves; the term “octave” is borrowed from acoustics and has the same meaning.

The use of a logarithmic scale offers great conveniences, as was already pointed out by Rubens:

a) it is absolutely symmetrical with respect to wavelengths and frequencies; it makes it possible to avoid excessive expansion of the scale toward one or the other end of the spectrum;

b) it corresponds exactly to the division into octaves adopted in acoustics; one and the same absolute difference always corresponds to one and the same ratio of wavelengths and frequencies.

The first graphical representation of the spectrum on a logarithmic scale was proposed by Rayleigh in 1883 (England); this method was used by Gouy in 1889 (France), Rubens in 1900 (Germany), and Lebedev in 1901 (Russia). In connection with the filling of the last gap in the scale of electromagnetic waves, a detailed scale of electromagnetic waves was compiled by Glagoleva-Arkadieva in 1926 (USSR). At present, diagrams of the electromagnetic spectrum are often given in textbooks, books, journals, manuals, Soviet and foreign.

For a survey of the whole picture of the complete electromagnetic spectrum, Fig. 1 presents a scale of wavelengths and frequencies of oscillations of the electromagnetic spectrum.

On the lines \(A, B, C\) are indicated: on \(A\)—wavelengths \(\lambda\) in various units; on \(B\)—wavelengths \(\lambda\) in cm; on \(C\)—the corresponding frequencies \(f\) of oscillations. On the bands \(D, E, F, G, H, I, K\) are marked: on \(D\)—the logarithmic scale in octaves; on \(E\)—the kinds of radiation—a band consisting of two separate bands superposed on one another with adjacent ends; on \(F\)—methods of obtaining waves; on \(G\)—radiators; on \(H\)—the surnames of authors who obtained waves and oscillations in various regions of the scale; on band \(I\)—the names of the regions of the wave scale according to the present draft standard; on \(K\)—the terms for the parts of each wave region, practical and metric. The wave scale covers more than 80 octaves; the number of octaves \(n\) in any interval of wavelengths, for example from \(\lambda_1\) to \(\lambda_2\), is calculated by the formula:

\[ n=\frac{\lg \dfrac{\lambda_2}{\lambda_1}}{\lg 2}; \tag{1} \]

the number of octaves in the interval of the whole electromagnetic spectrum will be more than

\[ N=\frac{\lg \dfrac{420\cdot 3\cdot 10^{10}}{10^{-11}}}{\lg 2}=80\ \text{octaves}, \tag{2} \]

where \(420\cdot 3\cdot 10^{10}\ \text{cm}\) is the wavelength corresponding to the lowest-frequency currents used in the laboratory (infra-low frequencies), the measure of whose frequency is conventionally taken as a period of 7 min, and \(10^{-11}\ \text{cm}\) represents the length of a very short wave of the electromagnetic spectrum—gamma rays of \(1\ \mathrm{X}=10^{-11}\ \text{cm}\).

It is not difficult to see that the scale of electromagnetic waves and oscillations shown here (Fig. 1) makes it possible to determine the wavelengths and frequencies of oscillations not only in octaves, but also in powers of 10. The sequence of the repeated series of quantities \(1, 2, 4, 8, 16, 32, 64, 125, 250, 500, 1000\) was borrowed from P. N. Lebedev (1901). This series covers an interval of approximately 10 octaves \((2^{10}=1024)\).

§ 3. Justification of the Need for Standardization of Terms and Classification of Frequencies and Wavelengths of the Complete Electromagnetic Spectrum

Electromagnetic waves are used in radio engineering and X-ray engineering, soil science and geology, biology and medicine, metallurgy, aviation, and in many other scientific and technical disciplines and branches of industry. Representatives of various specialties who, in their work, use the same portions of the electromagnetic spectrum need a common language for rapid and clear mutual understanding. Therefore, the standardization now being carried out by the All-Union Committee for Standardization under the Council of Labor and Defense must, in a timely manner, encompass this field of knowledge in all areas of science and technology.

§ 4. Draft Standard

The draft has been drawn up according to the following plan:

A. Name of the standard:

Scale of frequencies and waves of electromagnetic oscillations; classification and terms.

B. Sections:

I. Names of frequencies of oscillations.
II. Frequencies in derived units and in hertz.
III. Names of regions of the wave scale.
IV. Names of waves (rays) in portions of regions.
V. Wavelengths in derived units and in cm.
VI. Number of octaves.

The draft standard prepared according to this plan is presented in Table 1.

§ 5. Explanations to the Draft

The basic principles in preparing this draft were: a) the use of classifications and terms already in practical use in the Union and abroad, taking account of their industrial significance; b) clarification or replacement, as well as supplementation with new ones for logical linkage of the standard over the entire scale of electromagnetic waves. These provisions should make it possible for the standard to be easily mastered, for practical handling of it to be simple, and for the standard to be as acceptable as possible in the possible standardization of terms for waves and oscillation frequencies on an international scale.

The principle of classifying frequencies and waves based on the relation of the latter to some substance (for example, the division of radio waves into short, ultrashort, etc., according to their relation to the earth’s atmosphere, to sea water, etc.) or according to the method of measurement (for example, measurements in the region of long X-rays by a vacuum spectrograph with a crystal or by a vacuum spectrograph with a ruled grating, etc.) was deemed incorrect because of the contentiousness of the question of selecting “standard” substances and methods of measurement.

The draft standard contains 6 sections (vertical columns, Table 1), in which the regions of the scale are characterized. The entire scale of frequencies and waves consists of 8 regions, characterized chiefly by the principal methods of exciting electromagnetic oscillations; for those of them for which their frequency characteristics come to the fore (infra-low and low frequencies, industrial frequencies, audio fre-

cies), their names are absent in Section IV; for very high frequencies (infrared rays, light rays, ultraviolet rays, X-rays, gamma rays), on the contrary, the characteristics more commonly used for them as rays or waves are given.

Section I

In Section I the names of oscillation frequencies are given; this characteristic comes to the fore for the first four portions of the first region of the wave scale, since long-lasting oscillatory processes are usually characterized by the frequency of oscillation, and not by the length of the wave corresponding to these oscillations. These are infralow frequencies, low frequencies, industrial frequencies, and sound frequencies.

Section II

Section II contains the oscillation frequencies, expressed in hertz and in larger units proposed in the draft of the Swiss Commission on Standardization:

\[ \begin{aligned} \mathrm{kHz} &= \text{kilohertz} = 10^3 \text{ hertz},\\ \mathrm{MHz} &= \text{megahertz} = 10^6 \text{ hertz},\\ \mathrm{GHz} &= \text{gigahertz} = 10^9 \text{ hertz},\\ \mathrm{THz} &= \text{terahertz} = 10^{12} \text{ hertz}, \end{aligned} \]

In order to cover the enormous range of frequencies of electromagnetic oscillations of the entire electromagnetic spectrum, the units indicated above are supplemented by:

\[ \begin{aligned} \mathrm{kTHz} &= \text{kiloterahertz} = 10^{15} \text{ hertz},\\ \mathrm{MTHz} &= \text{megaterahertz} = 10^{18} \text{ hertz},\\ \mathrm{GTHz} &= \text{gigaterahertz} = 10^{21} \text{ hertz}. \end{aligned} \]

Section III

This section contains the names of the regions of the scale.

I. Low-frequency waves.
II. Radio waves.
III. Ultraradio waves.
IV. Infrared waves (rays).
V. Light waves (rays).
VI. Ultraviolet waves (rays).
VII. X-ray waves (rays).
VIII. Gamma waves (gamma rays).

The names found in this section have been assigned to the regions of the scale as already established in world literature; for example, see the pages containing scales of electromagnetic waves in the following books:

An outline of atomic physics, O. Blackwood and oth., New-York 1933.
The development of physical thought, L. B. Loeb and A. S. Adams, N. Y. 1933.
Grimsehls Lehrbuch der Physik. Berlin 1932.
Physik, W. H. Westphal. Berlin 1933.
L’énergie rayonnante, A. Forestier. Paris 1926.
and others.

In order to give an idea of the agreements and disagreements in world literature on this question, below are given some names of regions borrowed from Soviet and foreign literature.

1. Region of radio waves — Radiowaves; Ondes hertziennes; Radiowellen.
2. Region of hertzian waves — Hertzian waves; Ondes hertziennes; Hertzsche Wellen.

Scale of Frequencies and Waves of Electromagnetic Oscillations

Classification

\[ \begin{aligned} \mathrm{Hz} &= \text{hertz} = 1 \text{ oscillation per second},\\ \mathrm{kHz} &= \text{kilohertz} = 10^3\ \mathrm{Hz},\\ \mathrm{MHz} &= \text{megahertz} = 10^6\ \mathrm{Hz},\\ \mathrm{GHz} &= \text{gigahertz} = 10^9\ \mathrm{Hz},\\ \mathrm{THz} &= \text{terahertz} = 10^{12}\ \mathrm{Hz},\\ \mathrm{kTHz} &= \text{kiloterahertz} = 10^{15}\ \mathrm{Hz},\\ \mathrm{MTHz} &= \text{megaterahertz} = 10^{18}\ \mathrm{Hz},\\ \mathrm{GTHz} &= \text{gigaterahertz} = 10^{21}\ \mathrm{Hz}. \end{aligned} \]

\[ \begin{aligned} \text{km} &= \text{kilometer} = 10^3\ \text{m},\\ \text{m} &= \text{meter} = 10^2\ \text{cm},\\ \text{mm} &= \text{millimeter} = 10^{-1}\ \text{cm},\\ \mu &= \text{micron} = 10^{-3}\ \text{mm} = 10^{-4}\ \text{cm},\\ \text{\AA} &= \text{angstrom} = 10^{-8}\ \text{cm},\\ \text{X} &= \text{x-unit} = 10^{-11}\ \text{cm}. \end{aligned} \]

1. Infrared waves — Infrared or heat rays; Ondes infrarouges; Ultrarot; Infrarot;
2. Ultraviolet rays — Ultraviolet; ultraviolettes; Ultraviolette.
3. X-rays — X-rays; Rayons X; Röntgenstrahlen.
4. Gamma rays — γ-rays; Rayons γ; γ-Strahlen.

Sections IV and V

These sections contain subdivisions of the above-mentioned regions into groups, the names (IV) and classification (V) of the latter. In the first region of low-frequency waves, no names are given for the groups of waves, since their more commonly used characteristics are given in Section I. In Section V are found the wavelengths expressed in derived units and in cm.

In the 2nd region—the region of radio waves—the names and classification of the waves are taken from the standard recommended by the resolution of the International Committee at The Hague in 1929.

In the 3rd region—the region of ultraradio waves—the names and classification of waves encountered in the literature have been adopted; one name is new: “transitional ultraradio waves,” a name justified by its internal meaning: in this portion of the spectrum a transition takes place from the radiation of an individual Hertzian vibrator to the radiation of molecules and atoms of matter.

In the 4th region—the region of infrared waves—there are groups: decamicron and micron waves. Such a subdivision and such names are convenient because of their definiteness and simplicity.

Region 5 contains light waves. The names and classification of these rays are taken according to data proposed by the photometric laboratory of VIMS (the laboratory’s review of the present project).

Region 6—ultraviolet waves (rays)—is divided into 2 parts: the first—the nearest ultraviolet rays, including the region of Schumann and Lyman, up to 500 Å, and the second region—the extreme ultraviolet rays, from 500 to 50 Å, covered by the new works of Siegbahn and his school; this region will be discussed further below.

Region 7. X-ray waves (rays). This region, over a large extent, overlaps the two neighboring regions: on the one hand, the extreme ultraviolet rays from 500 to 50 Å; on the other, gamma rays from 400 to 40 X. The classification and naming of the rays in this region of the scale are the most difficult. In the project, emphasis is placed on the practical significance of X-rays: the most extreme X-rays, which are not used in technology and have great scientific significance in the development of the general theory of the spectra of matter, are assigned to the region overlapping with the extreme ultraviolet rays (from 500 to 50 Å): there they have the name “extreme ultraviolet rays.” The interval from 50 to 1 Å, corresponding to the upper limits of the continuous X-ray spectrum of “white” radiation (from 0.2 to 12 kV), directly adjoins the boundary of the ultraviolet rays; this justifies its name “boundary X-rays” (Grenzstrahlung). The next interval, from 1 to 0.4 Å, corresponding to the upper limits of the continuous X-ray spectrum from 12 to 31 kV, has the name “soft X-rays”: in practice this name is often applied to X-rays used for fluoroscopy and radiography of light materials, extremities of the human body, regions of superficial therapy, etc. X-rays possessing wavelengths greater than 0.4 Å or less are encountered together with the longest gamma rays and overlap with them from 0.4 Å (400 X) to 40 X.

40 X corresponds to the upper boundary of “white” radiation at a voltage on the tube of 300 kV, which in most cases is used for technical purposes, although in individual cases X-rays at tube voltages up to 600 and 650 kV are also employed. This interval from 400 to 40 X, being common to X-rays and

gamma rays, is widely used in technology, biology, and medicine as X-rays, i.e., rays obtained by means of X-ray tubes; they are used in radioscopy and radiography of machine parts, parts of airplanes, metal specimens, and various articles, in X-ray diagnosis and X-ray therapy in medicine, etc.; in practice the indicated portion of the scale is sometimes divided into “medium” and “hard” X-rays. In view of the difficulty of drawing boundaries between these portions, in the present draft the general name “hard” X-rays has been proposed.

The 8th region is gamma waves (gamma rays); it comprises the interval from 40 to 1 X, containing two groups of rays: Decan X gamma rays and X gamma rays. The division into these two groups and the names of the latter are justified by the simplicity of remembering the names.

Section VI

Section VI contains the frequency and wavelength intervals of each region of the scale, expressed in octaves.

LITERATURE

1. Lord Rayleigh, Nature XXVIII, 559, 1883. 2. Guillaume Ch. Ed. Revue Générale des Sciences, p. 5. 1899. 3. Rubens H. Le spectre infra-rouge, Rapporte, présenté an congres international de Physique, reuni à Paris en 1900. 4. Lebedev P. N. Scale of electromagnetic waves in the ether, Fizich. obozr., 101; collected works, 1912. 5. Glagoleva-Arkad’eva A. A., A new scale of electromagnetic waves. Uspekhi Fizich. Nauk, vol. VI, 16, 1926. 6. Landolt-Börnstein, Phys.-Chem. Tabellen, B. 11, 807 1923. 7. Moyer and Wostrel, The Radio Handbook. New York and London, 1931. 8. Müller-Pouillets, Lehrbuch d. Phys., 11, 397, 1909. 9. Yur’ev M. Yu., Theory of telephone transmission and its practical application, Moscow, 1927. 10. Holweck F., De la lumière aux rayons X. Paris, 1927. 11. Loeb L. and Adams A. S. The Development of Physical Thought. New York 1933. 12. Grimseis Lehrbuch der Physik, Berlin 1933. 13. Westphal W. H., Physik, Berlin, 1933. 14. Forestier A., L’énergie rayonnante. Paris 1926. 15. Blackwood L. and others, An Outlines of Atomic Physics. New-York 1933. 16. Technical Encyclopedia. 17. Galanin D. D., Electromagnetic waves, 1934. 18. Irving Saxl, Zs. f. Techn. Phys. 15, 1934. 19. Glagoleva-Arkad’eva A. A. The complete electromagnetic spectrum, Sorena No. 1, 32, 1935. 20. Periodical literature on this question.