  We report on the observation of optical activity of quantum wells resulting
in the conversion of the light polarization state controlled by the light
propagation direction. The polarization conversion is detected in reflection
measurements. We show that a pure $s$-polarized light incident on a quantum
well is reflected as an elliptically polarized wave. The signal is drastically
enhanced in the vicinity of the light-hole exciton resonance. We show that the
polarization conversion is caused by the spin-orbit splitting of the light hole
states and the birefringence of the studied structure. The bulk inversion
asymmetry constant $\beta_{h} \approx 0.14$ eV\AA, is determined for the ground
light hole subband in a 10 nm ZnSe/ZnMgSSe quantum well.
