The Light Scattering by Elongated and Flattened Spheroids

                         N.V. Voshchinnikov
        Astronomy Department and Sobolev Astronomical Institute,
                     St.Petersburg University
                               and
                         V.G. Farafonov
         St.Petersburg University of Aerocosmic Instrumentation


                            Abstract

The first attempt to study the optical properties of very elongated
(cigar-like) and very flattened (pancake-like) spheroidal particles
 with sizes larger than the wavelength was undertaken.
We use the method of separation of variables [1] and new ideas in the
calculations of the spheroidal wave functions [2].
The behaviour of the extinction cross-sections and phase function
with size parameter is considered for particles
with aspect ratios up to $a/b = 100$ or more.

Firstly, the method was applied to calculations of the optical properties
of non-absorbing spheroids. As the extinction and scattering
cross-sections were the same (with 6 and more digits),
the values presented are expected to be correct.

The geometry of the phase function of scattered radiation in forward/backward
and left/right directions is studied. As the aspect ratio increases, the
scattered radiation concentrates on the cone surface like for infinite
cylinders (prolate spheroids) or in the directions of refracted and
reflected rays (oblate spheroids).

The quasi-static approximation for extremely prolate and oblate spheroids
and the range of its validity are discussed.

The work was partly supported by INTAS (grant 99/652).

                    References:

[1] N.V. Voshchinnikov, V.G. Farafonov, Optical properties of spheroidal
                        particles, Astrophysics and Space Science, 204,
                        19, 1993.

[2] V.G. Farafonov, N.V. Voshchinnikov, New method of calculations of
                        spheroidal wave functions: Jaff\'e expansions,
                        2000, in preparation.