Anisotropic incident EM radiation is rescattered by a charge sitting on spacetime ripples (i.e., GWs) and becomes polarized. The GWs propagate along the line of sight while being polarized in a plane perpendicular to the the line of sight, and the rescattered (outgoing) EM radiation is induced by the GWs. Note that the directions of propagation and polarization of the GWs are fitted into the observer’s reference frame; with propagation along the z-axis (coinciding with the line of sight) and polarization along the x-axis and the y-axis. (Credit: CAPMAP website, http://quiet.uchicago.edu/capmap; reproduced with some modifications.)