Extinction of Light by Suspensions of Silica Dust

Application of approximations relating to spheres to the scattering of light by polydisperse dust clouds is discussed theoretically. If the relative refractive index of the particles is low, the shapes of the particles are not extreme, the particles are not orientated, and only the scattering forward through angles less than 30° is considered, the scattering seems likely to resemble that by clouds of spheres of the same sizes. Forward scattering and extinction by carefully fractionated silica dusts suspended in liquid were measured using an extinction method, and for particles larger than 2 microns showed good agreement with Mecke's approximation. For smaller particles absolute extinction measurements were not made, but relative values indicated the presence of aggregates. The aggregates seemed to diffract like opaque discs, and the unaggregated particles seemed to scatter like Rayleigh-Gans spheres.