Auto Glass: Passenger Side Mirror, passenger mirror, replacement mirror glass

Question
I recently hit a parked car and knocked the glass right off of my passenger mirror. I was wondering if I had to buy a whole new arm and mirror or just the glass part.

Answer
Check to see if the backing plate is still there, or if it came off with the glass.
The backing plate is what the glass is attached to.  The glass and backing usually snap into the mirror assembly and cover up the motor of the mirror.
Depending on the make and model of your vehicle there are replacements that you can purchase usually for a fraction of the cost and effort of replacing the complete mirror head.
Here is a link that explains further:
http://www.automirrorglassonly.com/catalog.php/automirrorglass/pg10371
Again, if you see the motor of the mirror that actually moves the mirror, then you are missing the backing plate.
Hope this helps.