Abstract

Light scattered from a fluid surface has a spectrum that involves lines slightly displaced from the incident frequency. The detection of these lines requires resolution far beyond that of normal spectrometers. Laser heterodyne beat frequency techniques developed recently allow resolution of Brillouin lines down to the low audio-frequency region. We have developed a simple spectrometer that detects the Brillouin spectra that are associated with the interaction of light and random surface ripples. The theory and experimental techniques involved are discussed. The capillary ripple dispersion equations have been solved numerically in order to provide theoretical estimates of the frequency of the splitting and estimates of the line shape. A doublet in the photocurrent power spectrum was predicted and was observed with increasing wave number for the air-water interface. The Brillouin shift observed for the air-water interface obeyed the Kelvin dispersion relation. Applications of this new technique to the study of interfacial forces are discussed.

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