This article is from the Disney's Haunted Mansion FAQ, by Don Bertino bertino@netcom.com with numerous contributions by others.
The mirrors that you are looking into are one-way mirrors or 50% mirrors.
They are the ones security use to check shop lifters or police use in
interrogation rooms (I know I have been watching too much TV :-) The
ghosts are in sync with the doom buggy going by and have a weak light
put on them. This allows you to see both your reflection and the ghosts
thru the mirror.
 
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