Anisotropic incident EM radiation is rescattered by a charge sitting on spacetime ripples (i.e., GWs) and becomes polarized. Here the incident and outgoing EM radiation fields and the GWs are illustrated with respect to the source frame ﹩({x}^{\prime },{y}^{\prime },{z}^{\prime })﹩: the GWs propagate along the ﹩{z}^{\prime }﹩-axis while being polarized along the ﹩{x}^{\prime }﹩-axis and the ﹩{y}^{\prime }﹩-axis, and the rescattered (outgoing) EM radiation induced by the GWs should be expressed in the same frame. Concerning the relationship between the source frame ﹩({x}^{\prime },{y}^{\prime },{z}^{\prime })﹩ and the observer’s frame ﹩(x,y,z)﹩, we note the two cases: (i) the propagation direction of the GWs does not coincide with the line of sight, i.e., ﹩{{\boldsymbol{e}}}_{z}^{\prime }\ne {{\boldsymbol{e}}}_{z}﹩, (ii) the propagation direction of the GWs coincides with the line of sight, i.e., ﹩{{\boldsymbol{e}}}_{z}^{\prime }={{\boldsymbol{e}}}_{z}﹩. (Credit: CAPMAP website, http://quiet.uchicago.edu/capmap; reproduced with some modifications.)