Light Scattering by Large Ellipsoidal Particles I. Rayleigh-Debye Approach JOSIP J. PETRES AND GJURO DEZELIC Department of Physical Chemistry, Institute "Rug jet Bo~kovig" and Department of Biocolloidal Chemistry, Andrija Stampar School of Public Hea#h, Faculty of Medidne, University of Zagreb, 41000 Zagreb Croatia, Yugoslavia Received April 15, 1974; accepted May 24, 1974 The angular dependence of the light scattered by monodisperse barium sulfate particles was determined at wavelengths of light 546 and 436 nm. The particles had the shape of prolate ellipsoids of revolution with different axial ratios. The results are discussed on the basis of the Rayleigh-Debye theory, and experimental and theoretical values of P (0) factors for ellipsoidal particles are compared. INTRODUCTION At the present time there is no exact general theory, like Mie's theory for isotropic spherical particles, for the calculation of the intensities of the light scattered by randomly oriented ellipsoidal particles of arbitrary size and refractive index. Formally, the problem of the scattering of an electromagnetic plane wave by a triaxial ellipsoid was solved long ago by M6glich (1), but the explicit solutions of pertinent differential equations have not been obtained. In most papers light scattering by ellipsoidal particles was treated theoretically on the basis of the Rayleigh theory for ellipsoids small in comparison with the wave- length (2) or on the basis of the Rayleigh- Debye theory which is applicable to particles with a refractive index sufficiently close to that of the surrounding medium. A brief discussion on the problem, especially concern- ing the light scattering by ellipsoids compar- able to the wavelength, can be found in Kerker's book (3), or in papers of Stevenson (4) and Shatilov (5). The application and the range of validity of the Rayleigh-Debye theory were tested extensively in the case of spherical and cylindrical particles by comparison of results obtained using the approximate formulae with values calculated by the exact theory (see e.g., Ref. (3)). However, there are rather few papers reporting on absolute measure- ments of the angular light scattering on ellipsoidal particles having dimensions and refractive indices as required by the Rayleigh- Debye theory. Several years ago Koch (6) examined the applicability of the Rayleigh-Debye approach using suspensions of mitochondria and bacteria as model systems for ellipsoidal particles. Later experiments (7, 8) have shown that the Rayleigh-Debye approximation for calcu- lating the intensity of scattered light at angles up to 30 ° applies relatively well to particles as large as bacteria (7), but a significant discrepancy was found between the theory and experimental measurements at high angles. Recently, Cross and Latimer (48) have used the Rayleigh-Debye approximation for both the homogeneous and the coated prolate ellipsoid to interpret the angular light scatter- ing (10-90 °) from randomly oriented systems 296 Journal of Colloid and InterfaceScience, Vol. 5;0, No. 2, February 1975 Copyright © 1975 by Academic Press. Inc. All rights of reproduction in any form reserved.