Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: science_guy on 16/02/2007 16:20:15

Title: How do one-way mirrors work?
Post by: science_guy on 16/02/2007 16:20:15
How do one way mirrors work? 

Do they actually let light through, but reflect the other way? 

What if you put one way mirrors facing solar panels, so the light that escapes by being reflected from solar panels will go back to be absorbed? Should I put a patent on this before someone steals it?
Title: Re: How do one-way mirrors work?
Post by: eric l on 16/02/2007 17:32:58
If my memory does not fail me here, the "one way mirrors" reflect light in the direction where the light is strongest.  The light outside the solar panel will always be stronger than the light reflected by the back of the panel, so, no gain to be expected.
What actually happens in a one way mirror is that the reflecting coating is very thin, and lets light through.  If your room is in the dark, the light from your room reflected by the mirror will be overrun by the light from the other room that is transmitted through the reflecting coating.
(What I remember on "one way mirrors" is from the time they were used in the visors of movie cameras : you had to put your eye close to the eyepiece so that no light from behind could penetrate and eventually touch the film)
Title: How do one-way mirrors work?
Post by: chris on 17/02/2007 22:03:22
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!

Title: How do one-way mirrors work?
Post by: daveshorts on 19/02/2007 11:11:22
This is exactly the same as why you can see your reflection on the inside of your bedroom window with the light on at night, but you can't during the day.

A piece of glass is a very bad mirror, it will transmit maybe 95% of the light that hits it and reflect 5%. Normally the transmitted light overwhelms the reflected light, so you don't notice the reflection, however if there is no transmitted light (it is dark on the other side) all you can see is the reflection.
Title: How do one-way mirrors work?
Post by: chris on 20/02/2007 21:45:34
Dave - why do you think that when you tilt down the rear-view mirror in the car (say someone behind has very bright lights) you can still see a (now much fainter) pair of headlights in the glass of the tilted mirror?

Title: How do one-way mirrors work?
Post by: daveshorts on 20/02/2007 23:18:50
because they mount a piece o glass at an angle in front of the mirror, normally only the ceiling is reflected in it, and the reflection is so weak you don't notice it. However when you flick the main mirror away it ends up at the correct angle to reflect lights behind you badly, so not dazzling you. The good reflection fron the mirror is now looking at the floor. There is normally a toggle to move the mirror the right amount to swap between the two.

Personally I don't like it as it means you can't see as much in the mirror, and you are less aware of cars behind you.