The scattering of light by liquid boundaries and its relation to surface tension. —Part II

Abstract

In the first paper, a description was given of the scattering of light by metallic liquid surfaces, particularly of the manner in which the intensity and state of polarisation of the scattered rays vary with the angle of incidence of the primary rays and the direction of observation. We now proceed to consider the phenomena observed when the clean and dust-free surface of a transparent liquid is strongly illuminated. Whereas in the case of metals we have a very few substances which are liquid at ordinary temperatures, an enormous variety of transparent liquids is available for the purpose of the present study. In fact, at the time the investigation was taken up, an extensive collection of pure organic chemicals had been obtained from Kahlbaum, and bulbs containing some 64 different liquids, rendered dust-free by repeated distillation in cacuo , were ready for a programme of quantitative studies of the internal lightscattering. This collection naturally proved very convenient also for the purpose of the comparative study of the surface-scattering, and the extended observations made possible by its aid served to bring out very clearly the influence on the phenomenon of the surface tension of the liquid, and thus to establish its molecular nature. As already remarked in the first paper, in the case of transparent fluids, the surface-scattering is accompanied by the internal-scattering within the liquid when a pencil of light is concentrated upon the surface, but the two effects are distinguishable from each other in several particulars. By using a good achromatic lens to focus a well-defined image of the sun on the boundary, the surface opalescence appears as a sharply bounded circular or elliptic disc of light, whose aspect varies very much with the direction of observation while that of the internal-scattering does not. The colour of the surface opalescence is also much less blue than that of the internal-scattering, and, indeed, by contrast with it appears nearly white. Green, yellow and red filters held in front of the eye diminish the brightness of the volume effect much more (in increasing order) than they do that of the surface effect, and hence assist greatly in studying or photographing the latter phenomenon. The brightness of the surface-scattering also varies with the direction of observation, while that of the internal-scattering in dust-free liquids is practically invariable. In the case of oblique incidence of the primary beam, the surface-opalescence is conspicuously brighter when viewed in directions adjacent to those of the reflected or transmitted pencils than in other directions. In fact, it then stands out very clearly, and may be distinguished even with liquids such as carbon disulphide or nitrobenzene, in which the internal-scattering is so strong that it usually overpowers the surface effect.