Re: One-way mirrors

A one way mirror is made by coating a dark piece of glass, such as
smoked glass or 'green' glass with a very very light coating of reflective
material, usually aluminum alloys (coating technology has changed since I
played with mirroring back in my telescope days).  Anyway, the coating
of reflective material is light enough to be translucent to light 
(allowing some light to pass through, sort of like sun glasses), but since
it is reflective, some of the light is reflected back, thus, a mirror.
Since the glass behind the mirrored coating is dark glass or smoked
glass, there isn't enough available light to pass through both the 
smoked glass and the translucent mirrored coating.  Thus, on the other
side all you see is your own reflection.  You can increase the lighting
behind the glass, and then there will be enough available light to pass
through both the smoked glass and the mirrored coating - at which point
you can see both ways through the one way mirror.

So, in summary, a one way mirror is basically a very, very thin film of
reflective material, much thinner than a regular mirror, coated onto
a piece of darkened glass.