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One way mirrors are mythical mirrors - they don't exist. If they did you'd potentially have a perpetual motion machine on your hands (to find out why, read on).We've all seen the "good guys" secretly watching the "bad guys" in the next room through such a mirror, but if they don't exist, where did the idea come from. Well, although you can't obtain a genuine one-way mirror, you can make an approximation to one, and here's how it works.You take a pane of glass and apply a very thin coating of reflective material to it. Usually aluminium or silver is used. This layer is so thin that it lets some light pass through, say 20%, although it reflects the majority of the light hitting it (the remaining 80%) on either side.This pane of glass is then placed between two rooms, one room where the "good cops" will sit and the other room where the "baddies" will be observed. Then, and this is the most important part, you alter the light levels in the two rooms - the "good cops" need to be very dark, whilst the "baddies" need to be extremely well lit (as a general rule at least 10 times brighter than the light in the "good cops'" room).The net result is that on either side of the glass you see your reflection plus the light coming through from the other side. Because the "good cops" are sitting in relative darkness, the amount of light coming through from the "baddies" side is much brighter than their own reflection, so this is mainly what they see. For the "baddies" on the other hand, because their room is so much brighter, the traces of light filtering through from where the "good cops" are sitting is totally obscured by the light in the room, so the only thing visible is the reflection from the room itself.The best analogy I have come across to explain this is to imagine that in one of the rooms the people are shouting at each other and making a lot of noise, whilst in the other room the people are whispering to one another. As a result, the people whispering will be able to hear the noise of the rowdy lot next door, but the noisy bunch will not be able to hear the whisperers because their own noise will have drowned out the sound.So where does that perpetual motion machine idea come in? Well (and I'm grateful to Dr Karl for telling me about this), if you had a genuine one-way mirror you could make a box from it so that light could get into the box, but not out again (because it would be bouncing about all over the place inside the box, reflecting off the mirror surfaces). Since light is energetic you would have a box that collected energy without you having to do any work. In essence, if you tapped off the energy, you'd have a perpetual motion machine, which is a nonsense!Chris