Rayleigh-Gans-Debye applicability to scattering by nonspherical particles.

Rayleigh-Gans-Debye scattering theory is applicable to particles satisfying |m - 1| << 1 and 2ka |m - 1| << 1. It is often applied to large nonspherical particles such as bacteria where its validity is uncertain. The purpose of this study is to define the range of validity of the RGD approximation as applied to homogeneous nonspherical particles. Scattering calculations are made for a set of prolate spheroidal particles using the RGD approximation, and the results are compared to those obtained by the recently developed extended boundary condition method, a technique known to give accurate scattering results for nonspherical particles. Calculations for oriented particles with m = 1.05 verify that RGD error is dependent on particle orientation relative to the incident wave. Also, the error is found to increase with ka and to decrease with axial ratio for small particles, but increase with axial ratio for larger particles. Calculations for a particular randomly oriented particle show that the RGD approximation is more accurate for this case than if the incident wave is along the major dimension of the oriented particle.