This paper deals with the propagation of partially polarized light through media with birefringence and dichroism on the basis of vector equations and the resulting scalar equation. The vector equations are derived from general coupled-mode equations by the Stokes parameters transformation. The evolution of the polarization sate of partially polarized light is compared with that of polarization state of completely polarized light by using the unit Poincare sphere and the shrinking sphere. This propagation theory of absorption is applied to photoelastic media when they are absorbing.