I picked up my camera bag a few days ago, which was not zipped shut. Out fell two of my lenses. In the process I broke the focus motor on one lens and cracked a Hoya HD polarizing filter on the other lens. Looking at the damage to the polarizing filter I can see that only the front piece of glass (the part that spins) is cracked. The inner piece of glass is unharmed. I have three questions:
That's possible, but it doesn't sound likely, as that is the actual polarizing filter. Judging from filter prices, that is the most expensive part of the whole filter.
The back glass is a depolarizer. The only use for that (other than as a protective filter) would be if you put a linear polarizing filter in front of it, because then you get a circular polarizing filter.
If it's built the same as the Hoya polarizing filters that I have, there is a spring on top of the glass holding the glass in place. If you look around the edge you should see a small gap (about 3 mm) in the spring. I have removed the glass accidentally once or twice (by dropping the filter), so it should be possible to do on purpose. (Come to think of it, I actually assemled it wrong one time, so I had to remove the glass again, so I know that it's possible.)
I think photographically it's a rock now. But you can use it for doing things like polarizing source lights.