FROM CURRENT LITERATURE

Scattering of light by ultrasonic waves. In a vessel of rectangular cross-section, of great length and filled with a liquid, Debye and Sears excited, by means of piezoquartz, ultrasonic standing waves having a frequency of several million hertz. The wavelength of the resulting standing wave was tenths of a millimeter. Perpendicularly to the direction of propagation of the standing waves the authors sent a beam of light rays through a narrow slit. A lens, placed on the other side of the vessel, collected the beam and gave an image of the slit. When ultrasonic oscillations were excited, periodic changes in the density of the liquid arose, causing changes in its refractive index at the nodes and antinodes of the standing waves; because of this, the liquid acted like a diffraction grating, and instead of a simple image of the slit, diffraction spectra were obtained (up to the 20th order). From the values of the angles at which the spectra were obtained, it was possible to determine the ratio of the acoustic wavelength to the light wavelength and the velocity of ultrasound in the liquid. Experiments carried out with toluene and carbon tetrachloride gave values of the velocity coinciding with the theoretically calculated ones.

An analogous work was carried out by R. Lucas and P. Biquard (C. R. 194, 2132, 1932), working with a vessel filled with water.

A similar phenomenon in air was observed by E. P. Tawai (C. R. 191, pp. 92 and 988, 1931). He succeeded in obtaining excellent photographs on which standing ultrasonic waves are clearly visible. From his experiments he found the velocity of ultrasound in air to be 345 m/sec.

Finally, let us recall that visible sound waves were obtained by G. Kronig (Phys. Zs. 31, 908, 1930) two years earlier. He stretched a very thin tungsten wire (0.1 μm) along a tube. When standing sound waves were excited in the tube, a cooling of the wire was produced at the antinodes so considerable that it darkened, while a bright glow was preserved at the nodes. The maximum frequency of oscillations in these experiments was about 25 kilohertz ($\lambda = 1.3$ cm). (P. Debye and F. W. Sears, Proc. Nat. Acad. Amer. 18, 400, 1932).