Hello! I recently came across the Hanle effect, which is a reduction or increase in the polarization of the emitted light from an ensemble of atoms as a function of an externally applied magnetic field. Not so long ago I heard about the Faraday effect which is a rotation of the polarization of the light passing through an ensemble of atoms as a function of an external magnetic field (and it seems like there is a version for the case of an external electric field, too). I haven't learnt about these in my undergraduate EM or QM class and they are not something one can easily came across by chance (at least not the Hanle effect). I was wondering if there are other interesting effects like these where the light interacting with some atoms in the presence of some external field (or anything) changes some of its properties in a measurable way. Thanks!