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XXIV.—The Scattering of Light
Published online by Cambridge University Press: 15 September 2014
Summary
The present paper describes a number of experiments made in connection with Christiansen's experiment in which a beam of light is passed through a transparent insoluble powder immersed in a liquid, with the result that light of the particular colour for which the indices of powder and liquid are the same passes unaffected, while light of all other colours is scattered. In this communication only the simplest case of the above is dealt with, that, namely, in which a flat piece of glass, ground on one side, takes the place of the powder, the rugosities of the ground surface representing a single layer of grains, and air takes the place of the liquid. In such a case there is, of course, no colour of light for which the indices of solid and liquid are alike, and indeed it was found that the colour of the light made very little difference to the results. On the other hand, however, it soon became apparent that different ways of grinding the glass surface led to very different effects. Accordingly, as no previous work appears to have been done in this field, and as it seemed a promising one, it was decided to make a systematic study of the various cases. Each specimen of glass employed was photomicrographed, and had its polar light distribution measured by a photometer. Two methods of characterising the particular scattering power of a screen soon suggested themselves and have been formally defined—the one connected with the Angle of Maximum Total Emission, and the other with the Equivalent Cavity.
It is hoped to continue the investigation not only on the above lines, but also in the direction of ascertaining the effect of a number of plates, i.e. of successive layers of light-scattering particles; and in investigating the polarisation effects, which some rough preliminary experiments have shown to be marked.
I am glad of this opportunity of acknowledging the help I have received from the Trustees of the Carnegie Trust in the form of grants for the construction of the special apparatus necessary; and desire to tender my grateful thanks to Professor MacGregor for the many facilities for carrying on the work which he kindly placed at my disposal.
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- Copyright © Royal Society of Edinburgh 1914
References
page 264 note * Wied. Annal., Band xxiii, p. 298, Nov. 1884. Description by Christiansen of his original experiment.
page 264 note † “On an Improved Apparatus for Christiansen's Experiment,” Rayleigh, Lord, Phil. Mag., xx. p. 358, 1885CrossRefGoogle Scholar; also Nature, lx. p. 64, 1899.
page 265 note * E.g. SirAbney, W. de W., Jour. Soc. Chem. Ind., 31st July 1890, p. 772Google Scholar, “On the Accuracy of the Grease-Spot Photometer for Measuring the Density of Photographic Plates, and a Note on the Sector Photometer.” “A Demonstration of Scatter in Turbid Liquids,” Scheffer, W., Brit. Jour. Phot., p. 941, 9th Dec. 1910Google Scholar. Jones, C., Photographic Journal, 1898–1899, and André Callier, p. 200, 1909.Google ScholarTheory of the Photographic Process, Sheppard and Mees, pp. 38 and 114. See also The Brit. Jour. of Photography for 30th Aug. and 13th Sept. 1912, in connection with the scatter error in wedge photometry.
page 265 note † See, e.g., Electrical Photometry and Illumination, H. Bohle, p. 153 et seq.
page 265 note ‡ On the subject of diffuse reflection, see, e.g., Trotter, Illumination, its Distribution and Measurement, p. 93 et seq. Outlines of Applied Optics, P. G. Nutting, p. 165 et seq. Gilpin, F. H., Trans. Ill. Eng. Soc., v. 854–874, Dec. 1910.Google ScholarWright, H. R., Phil. Mag. (5), No. 49, p. 119, 1900.Google Scholar
page 265 note § See, e.g., Physical Optics, Wood, chapter on “Scattering by Small Particles.”
page 266 note * N2G2 was in all cases kept large enough for this to be very approximately true.
page 274 note * Following Lord Rayleigh, I tested the smoothness of the pits on the surface of the etched glass (no. 8) by using the pits as lenses, and the photomicrographs, Plate II. figs. 6 and 7, show the images thus obtained when the object was (1) a luminous point, and (2) a luminous cross. The definition is sufficiently good to leave little doubt as to the smoothness of the pits; and indeed the perfection of the images is somewhat surprising, in view of the fact that the pits are for the most part of a decidedly elongated shape, the length along the surface of the plate being often several times as great as the breadth, as appears clearly in the photomicrograph, Plate II. fig. 5. This fact, no doubt, accounts, partly at least, for the marked astigmatism shown by many of the images when thrown slightly out of focus.
page 275 note * Trans. Opt. Soc., vol. xi. p. 113, 1909–10.
page 275 note † Lord Rayleigh could find no evidence of surface flow in glass (loc. cit.), but other experimenters are satisfied of its existence (see “discussion” at end of Lord Rayleigh's paper); and it is well established as regards a great many other materials. See especially the Hurter Memorial Lecture given by Beilby, G., on “The Surface Structure of Solids,” Jour. Soc. Ghem. Ind., p. 1166, 1903.Google Scholar
page 277 note * We shall ignore the reduction of intensity in the transmitted light due to reflection losses, because in all the cases considered it is so very nearly the same. On making a calculation by means of Fresnel's formula, it appears that when a beam of light falls normally on the interface separating two media of refractive indices 1 and 1·5 respectively, 96·00 per cent, of the light energy is transmitted; and even when the obliquity of the beam has reached 30° to the normal, the percentage of energy transmitted is still as great as 95·85.
page 280 note * Curve no. 14 in fig. 6 shows the light intensity distribution that would be given by an infinitely long semicylindrical cavity, the axis of the circular cylinder being in the surface of the plate. Evidently this case can be calculated from the same equations (a) (b) (c) as before, if (c) be changed to i = dx/dθ. The results of the calculation are given in Table II. no. 14.