Apart from providing information on sources of radiation and scattering, polarization also probes the interstellar magnetic field in our galaxy as well as in radio galaxies via Faraday rotation. In some cases it can be difficult to determine how much of the Faraday rotation is in the external source and how much is local to our own galaxy, but in many cases it is possible to find another distant source nearby in the sky; thus by comparing the candidate source and the reference source, the results can be untangled.