A 'one way' mirror is actually a partially transparent mirror. Light reflects from this
sort of mirror, but some light also goes through it. In fact, ALL mirrors are partially
transparent, but manufacturers paint the back of the mirror with black paint.
To produce the 'one way' effect, the partial mirror is used to divide two rooms, then one
room is kept dark while the other room is brightly lit. People in the brightly lit room
see a mirror, since the image of the bright surroundings is reflected by the partial
mirror. People in the darkened room don't see their reflections, instead they see THROUGH
the mirror, and can watch the people in the brightly lit room.
If you suspect that a particular mirror is being used as a window, simply turn the lights
off in that room, then place a bright flashlight against the mirror surface. If there is a
hidden chamber behind the mirror, the flashlight will illuminate it, and since you're in a
darkened room, you'll see the hidden chamber.
A genuine 'one way' mirror is impossible to build because it violates the laws of
thermodynamics. If you made up a box of 'one-way' mirror material which was oriented so
more light flows into the box than flows out, then more and more light would flow into the
box and build up there. It takes energy to do such a thing. If genuine 'one-way' mirrors
existed, they would serve as energy sources, and you could use one to build a Perpetual
Answered by: William Beaty, B.S., Electrical Engineer, Seattle
'Our job in physics is to see things simply, to understand a great many complicated phenomena, in terms of a few simple principles.'