Open AccessThe scattering of light by liquid boundaries and its relation to surface tension. — Part III
Author(s) -
C. V. Raman,
L. A. Ramdas
Publication year - 1925
Publication title -
proceedings of the royal society of london. series a, containing papers of a mathematical and physical character
Language(s) - English
Resource type - Journals
eISSN - 2053-9150
pISSN - 0950-1207
DOI - 10.1098/rspa.1925.0123
Subject(s) - opalescence , surface tension , scattering , optics , light scattering , opacity , materials science , refractive index , chemistry , thermodynamics , physics
In Part II, the phenomena of the scattering of light by the surface of transparent liquids were described in detail. No reference was, however, made to the case of water, which stands in a special category owing to the exceptional properties of this liquid. Of all known transparent fluids at ordinary temperatures water has the highest surface-tension. Its refractive index is also low, and hence its surface-opalescence may be expected to be very feeble. Fortunately, however, the internal scattering in dust-free water is also very small, being in fact much less than that for any other known liquid, and hence, provided water is obtained dust-free and with an uncontaminated surface, there should be no difficulty in observing its surface-opalescence. This was actually found to be the case. By using water subjected to repeated slow distillationin vacuo in carefully cleaned pyrex glass bulbs, it can be obtained quite pure and dust-free. The surface-opalescence may then be observed, and in agreement with anticipation is found to be very feeble; it exhibits features of polarisation and intensity distribution in different directions very similar to those shown by other liquids. For inter-comparison of the surface-scattering power of different liquids, the following simple arrangement was adopted. A beam of sunlight from a large heliostat was used and reflected vertically upwards by a second mirror inclined at 45°. The beam passed through two lenses of equal focal length, which were placed as near together as possible and formed images of the sun upon the surfaces of the liquids contained in bulbs, placed one above each of the lenses. By varying the apertures of the lenses, the brightness of the opalescent areas on the two liquid surfaces could be varied till they were estimated to be of equal brightness, as seen by the eye placed below the surfaces at approximately the same angle. The angle of observation chosen was in each case slightly greater than the critical angle, so that the surface-scattering had maximum brightness. By taking the mean of a sufficient number of readings, fairly dependable measures could be obtained of the ratio of surface-brightness, at least in the case of liquids, for which the internal scattering was not so large as to interfere with the judgment of the eye.