  The spectral, temperature and magnetic field dependencies of the linear
dichroism in the reflection of light from the surface of an undoped single
crystal of La2CuO4 are studied. The changes of the dichroism absolute value and
its response to the external magnetic field induced by a weak optical
illumination were detected. The set of experimental results is analyzed from
the point of view of the high sensitivity of optical and magnetic properties of
cuprates to the density of charge carriers localized in CuO2 planes. It is
assumed that the poor reproducibility of some of the observed effects is
related to the proximity (on the x-T diagram of magnetic states) to the
antiferromagnet - spin glass phase boundary.
