Umov effect

Abstract: Total internal reflection occurs when light is incident on the interface of high- and low-refractive-index materials at an angle greater than the critical angle. Sunlight with high degree of linear polarization, such as atmospheric scattered skylight, can be converted with a high efficiency up to 53% to circular and elliptical polarizations by total internal reflection under water in the region outside Snell’s window. The degree of circular polarization is observed to be inversely dependent on the albedo of underwater objects and is shown to be a direct consequence of the Umov effect. Our results are important for underwater polarimetry, surveillance applications and studies of marine animals’ polarized vision near the water-air interface.

Abstract: The Umov effect manifests itself as an inverse correlation between the light-scattering maximum of positive polarization Pmax and the geometric albedo A of the target. In logarithmic scales, Pmax is linearly dependent on A. This effect has been long known in the optics of particulate surfaces and, recently, it was extended for the case of single-scattering dust particles whose size is comparable to the wavelength of the incident light. In this work, we investigate the effect of irregular shape on the Umov effect in single-scattering particles. Using the discrete dipole approximation (DDA), we model light scattering by two different types of irregularly shaped particles. Despite significant differences in their morphology, both types of particles reveal remarkably similar diagrams of log(Pmax) versus log(A). Moreover, in a power-law size distribution r-n with n=2.5-3.0, the Umov diagrams in both types of particles nearly coincide. This suggests little dependence on the shape of target particles in the retrieval of their reflectance using the Umov effect.