Fine Structure of the Rayleigh Line in Amorphous Substances

Abstract

As is well known, when monochromatic light scattered by a liquid is examined under high resolution it exhibits a fine structure: an undisplaced central line and two lines on either side with wavelengths slightly different from that of the incident light. The appearance of the displaced components was first predicted by Brillouin¹. On the basis of his theory, the observed displacements of frequency are regarded as a Doppler effect arising from the reflexion of the light wave by the progressive sound waves of thermal origin in the scattering medium. The frequency shift of the so-called Brillouin components is given by the formula where ν and c are the velocities of sound and light in the medium and θ is the angle of scattering. That the effect contemplated by Brillouin does arise in liquids and crystals is now a well-established experimental fact.