ABSTRACT
Electromagnetic radiation can become polarized by interstellar dust in three ways: through scattering, through extinction, and through emission. To explain the latter two phenomena, one has to evoke the presence of nonspherical grains, for example elongated particles shaped like cigars. They must also be aligned, otherwise the net polarization would be zero. The problem of computing the electromagnetic field in the interaction of light with an infinite cylinder for any ratio of cylinder radius over wavelength was first solved by Rayleigh for normal incidence and by Wait or oblique incidence. It is also possible to handle coated or multi-layered cylinders. In a cloud with anisotropic grains that are aligned to some degree, the extinction of a linearly polarized wave depends on the orientation of the electric field vector. The observed percentage of polarization rarely exceeds 10%. The upper limit is set by the difficulty of observing faint, strongly reddened stars.
