Subject: How does a two-way mirror work? How could I make one?

by two-way mirror, I mean something that you can see through like 
glass from one side (although it appears somewhat shaded) and 
from the other side looks like a normal mirror.  The kind used 
to hide security cameras.  I don't know if two-way mirror is the 
correct name for it, which may be why I can't find anything on 
the www.  
I'd like to turn the glass on a french door into a two-way mirror 
and install it for my front door.  I'm trying to find out wether 
mofifying the glass on an existing door, or building the door 
with mirrors would be more cost-effective.

Re: How does a two-way mirror work? How could I make one?