Scattering of light by bispheres with touching and separated components

We use the T-matrix method as described by Mishchenko and Mackowski [Opt. Lett. 19, 1604 (1994)] to compute light scattering by bispheres in fixed and random orientations extensively. For all our computations the index of refraction is fixed at a value 1.5 + 0.005i, which is close to the refractive index of mineral tropospheric aerosols and was used in previous extensive studies of light scattering by spheroids and Chebyshev particles. For monodisperse bispheres with touching components in a fixed orientation, electromagnetic interactions between the constituent spheres result in a considerably more complicated interference structure in the scattering patterns than that for single monodisperse spheres. However, this increased structure is largely washed out by orientational averaging and results in scattering patterns for randomly oriented bispheres that are close to those for single spheres with size equal to the size of the bisphere components. Unlike other nonspherical particles such as cubes and spheroids, randomly oriented bispheres do not exhibit pronounced enhancement of side scattering and reduction of backscattering and positive polarization at side-scattering angles. Thus the dominant feature of light scattering by randomly oriented bispheres is the single scattering from the component spheres, whereas the effects of cooperative scattering and concavity of the bisphere shape play a minor role. The only distinct manifestations of nonsphericity and cooperative scattering effects for randomly oriented bispheres are the departure of the ratio F(22)/F(11) of the elements of the scattering matrix from unity, the inequality of the ratios F(33)/F(11) and F(44)/F(11), and nonzero linear and circular backscattering depolarization ratios. Our computations for randomly oriented bispheres with separated wavelengthsized components show that the component spheres become essentially independent scatterers at as small a distance between their centers as 4 times their radii.