would take more than a blog post…. but I have found a nice flash animation that does a good job of showing how a 1/4 wave phase retarder can take a linearly polarised beam, and make it circularly polarised (or polarized for the Americans out there).
You can see it here , and there is a good text description too ( D Harrison University of Toronto)
Like most optical systems this is reversible, so it will equally take an incoming circularly polarised beam, and make it linear.
This is what happens with a CO2 laser, when a back reflection off a metal workpiece occurs, and the beam reflects straight back through the beam delivery system the “wrong way”, and possibly straight back into the laser.
At this point I imagine the Jude Law film “Enemy at the gates” where he shoots the sniper through his own rifle sight. Perfect alignment !