From the History of Physics

Device for Demonstrating Huygens’ Principle *)

P. N. Lebedev and I. F. Usagin

Those simple elementary experiments which serve as the basis of Huygens’ principle can be shown in a large lecture hall on water waves, if one uses, for their demonstration, the methods indicated by Becker ); for demonstrating diffraction phenomena the indicated method is unsuitable, since, even when using very large vessels, one has to resort to such short waves that an inexperienced observer, encountering the phenomenon for the first time, can no longer catch and follow their motion. These diffraction phenomena were demonstrated on small liquid surfaces by means of capillary waves by Vincent ) and Watson ***), who made photographic exposures under instantaneous illumination of the waves produced by means of an electric spark.

In order to demonstrate these instructive experiments in a large lecture hall not on previously prepared photographs, but to make it possible to observe the phenomenon itself directly as it forms, we used the stroboscopic method *), which makes it possible to follow the phenomenon in all its phases.

The arrangement of the experiments is evident from Fig. 1: an ordinary projection lantern \(B\), with the aid of a condenser \(C\), gives a real image of the crater of an arc lamp on a round diaphragm \(D\) \((\varnothing = 12\ \text{mm})\), fastened to an iron stand, onto which

) Publication and notes by A. Serdyukov.
) W. C. Backer, Phys. Rev., 10, p. 175, 1900.
) Vincent, Phil. Mag., 43 (1897), 45, 46 (1898), 48 (1899).
) Watson, Phys. Rev., 12, 276 (1901).
***) In the original, there is an obvious scribal error: it says “spectroscopic method.” (A. S.)

is placed, as is the lamp \(B\). Behind the diaphragm there rotates a disk made of aluminum, in which three slots have been cut*); this disk, by means of the pulley \(L\), is set in rotation by an electric motor, which gives the disk from 1000 to 2000 revolutions per minute.

The light, after passing through a cutout in the disk, falls on the lens \(C_1\), is reflected from the plane mirror \(S\), passes through the lens \(C_2\), through a shallow vessel with a bottom of mirror glass and the liquid poured into it; the lens \(C_2\) gathers the beam of rays on the filament, after passing which it is reflected from a plane mirror and falls on a vertical screen**).

Capillary waves on the surface of the liquid are excited by the electromagnet \(E\); on the axis of the rotating disk there is mounted an interrupter, which gives three interruptions of the current in the electromagnet during one revolution of the disk; in this way synchronism is achieved between the periods of the wave and of the illumination.

In order to obtain on the screen a picture not of standing waves in a definite phase, but of moving waves, the brush that makes contact with the interrupter is provided with the handle \(H\), and it can be turned by hand at the desired speed in the direction of rotation of the disk axis: one complete turn of the handle \(H\) corresponds to the displacement of all the waves by three of their wavelengths, and it is not difficult to choose such a speed of rotation as to obtain a continuous motion of the waves on the screen, slow enough for convenient observation.

*) The slots are calculated in such a way as to transmit about one-tenth of the fraction of the light; with such illumination the phenomenon is still very clearly visible in a darkened auditorium. If the slots are made larger, the intensity of the light increases somewhat, but at the same time the wave pattern becomes noticeably more blurred.

**) It is useful to take the diameters of the non-achromatic condenser lenses \(C_1\) and \(C_2\) no smaller than 25–30 cm. If smaller-diameter lenses are used, then, in order to obtain a sufficient number of waves in the field of view, one has to resort to very short waves, which are very strongly damped during their propagation and do not give a sufficiently sharp overall picture.

DEVICE FOR DEMONSTRATING HUYGENS’ PRINCIPLE

P. N. Lebedev and I. F. Usagin against the background of an image of waves obtained with the aid of the described apparatus.

*

Notes. 1. The present article was written by P. N. Lebedev in 1911 and was never published anywhere*).

1. The apparatus described here for demonstrating the propagation of waves was assembled and adjusted, on P. N. Lebedev’s instructions, by his student E. V. Bogoslovsky, with the active participation of I. F. Usagin. The striking experiments on wave propagation prepared by E. V. Bogoslovsky were demonstrated by him at the Twelfth Congress of Russian Naturalists and Physicians in Moscow and described in Fizicheskoe obozrenie, vol. XI, 1910.

2. The drawing of the apparatus that is absent from the manuscript of this article (see the figure) has been reconstructed from the original instrument, which is kept in the physics laboratory of Moscow State University. In addition, a photograph of P. N. Lebedev and I. F. Usagin has been added here against the background of an image of waves obtained with the aid of this apparatus.

*) Archive of the Academy of Sciences of the USSR, fond 293, inventory 1, no. 45.