Scattering of Electromagnetic Waves by Composite Spherical Particles: Experiment and Effective Medium Approximations

摘要

The differential scattering cross sections (phase functions I sub 2 2) and the normalized extinction and scattering cross sections (efficiencies) were measured for composite spherical particles. The size parameter x=2piR/Lambda was around 2pi. Composite spheres consisted of nonabsorbing matrix containing a small amount (1.6 and 2.7% by volume) of highly absorbing inclusions. Such composite particles may represent a realistic model of fog or cloud droplets containing small amounts of carbon or a composite atmospheric aerosol particle. Measured data is compared with those calculated using seven different effective medium approximations. The approximations of Bruggeman and Maxwell Garnett, the generalization of dynamic effective medium approximation derived by Chylek and Srivastava, and the experimental waveguide method of determination of the effective refractive index lead to an acceptable agreement between calculated and measured values. The reduced chi-square values for these approximations ranged between 0.6 and 2.0. The remaining three approximations (volume averages of refractive indices or dielectric constants and the Maxwell Garnett relation with matrix and inclusion materials interchanged) lead to reduced chi-square values between 4.0 and 12.0 demonstrating a large disagreement between calculated and measured scattering characteristics. Reprints. (JHD)