  We present a theoretical study of multi-mode scattering of light by optically
random media, using the Mueller-Stokes formalism which permits to encode all
the polarization properties of the scattering medium in a real $4 \times 4$
matrix. From this matrix two relevant parameters can be extracted: the
depolarizing power $D_M$ and the polarization entropy $E_M$ of the scattering
medium. By studying the relation between $E_M$ and $D_M$, we find that {\em
all} scattering media must satisfy some {\em universal} constraints. These
constraints apply to both classical and quantum scattering processes. The
results obtained here may be especially relevant for quantum communication
applications, where depolarization is synonymous with decoherence.
