What is a one way mirror and how does it work?

Asked by: Jesse


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 Motion machine.
Answered by: William Beaty, B.S., Electrical Engineer, Seattle