The physical-optics approximation and its application to light backscattering by hexagonal ice crystals

- The physical-optics Mueller matrix is a smoothed geometric-optics counterpart.
- Backscatter by partially oriented hexagonal ice plates has been calculated.
- Depolarization ratio for partially oriented hexagonal ice plates is negligible.

The physical-optics approximation in the problem of light scattering by large particles is so defined that it includes the classical physical optics concerning the problem of light penetration through a large aperture in an opaque screen. In the second part of the paper, the problem of light backscattering by quasi-horizontally oriented atmospheric ice crystals is considered where conformity between the physical-optics and geometric-optics approximations is discussed. The differential scattering cross section as well as the polarization elements of the Mueller matrix for quasi-horizontally oriented hexagonal ice plates has been calculated in the physical-optics approximation for the case of vertically pointing lidars.