  We resolve the existing controversy concerning the selection of the sign of
the normal-to-the-interface component of the wave-vector $k_z$ of an
electromagnetic wave in an active (gain) medium. Our method exploits the fact
that no ambiguity exists in the case of a {\em film} of the active medium since
its coefficient of reflectance is invariant under the inversion of the sign of
$k_z$. Then we show that the limit of the infinite film thickness determines a
unique and physically consistent choice of the wave-vector and the refractive
index. Practically important implications of the theory are identified and
discussed.
