[Heronumber0]

This may have been asked before but how do these mirrors work in Police Stations and so on to identify suspects from lineouts or to observe suspects in custody?

[paul.fr]

It's all to do with light, and lighting. This is what Chris wrote in a previous topic:

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

http://www.thenakedscientists.com/forum/index.php?topic=6560.msg68248

[Heronumber0]

Excellent answer which suits my simple mind very well. Thaank you for that.

[Heronumber0]

Oh, sorry about this - I just wondered about 'Dallas' (JR Ewing...) type buildings which have these 'two way mirrors'.  Does that mean that at night, on a particularly overcast night, you can see inside, if the lights are on? Or is the critical ratio of light inside to light outside still good enough to stop prying eyes?

[Atomic-S]

You can make a better one-way mirror as follows: Use a pane of smoky semitransparent glass instead of clear glass, and partially aluminize it on one side. Place the aluminized face toward the "bad" guys. Now, because the glass absorbs part of the light going through it, the light from the "good" guys that the "bad" guys will see is diminished below that with clear glass, whereas the brightiness of the
"bad" guys' reflection will remain the same. Result: The light on the "good" side can be brightened while keeping the same degree of invisibility which there was with the clear glass. On the "good" side, the light from the "bad" side is diminished by the semitransparency of the metallic layer, and additionally by the darkness of the glass, which seems to be disadvantageous. However, the reflection of the "good" guy's image is doubly attenuated, because their light must travel through the smoky layer twice. As a result, the overall directionality properties of the panel is improved over that with clear glass. With some adjustments in the values of metallization, this should permit greater separation of visibilities with the same lighting difference, or the same separation of visibilities under lesser lighting difference, which is desirable in that it would permit the "good" guys to light their office a bit brighter and still be assured that the "bad" guys could not see them easily.