Abstract
<p>It is shown that two stochastic electromagnetic beams that propagate from the source plane z=0 into the half-space z>0 may have different degrees of polarization throughout the half-space, even though they have the same sets of Stokes parameters in the source plane. This fact is due to a possible difference in the coherence properties of the field in that plane, but other reasons are also possible. The result is illustrated by an example.</p>
