# Thread: Is two-way mirror Maxwell's demon for photons?

1. Everybody has seen two-way mirror - transparent from one side, reflective from the second ... isn't that Maxwell's demon for photons?
Ok - it's not perfect - it absorbs some photons increasing own heat
and emits thermal photons - so it can stay in thermal equilibrium with
environment.

Let's take a container for photons (covered with mirrors), now place
two-way mirror in thermal equilibrium with photon gas inside, dividing
container into two parts.
The density of photons on the reflective side should be larger than on
the second - so it would reduce entropy?

2.

3. i think it has to do with polarity.
it lets light through one way, and blocks it the other way.

4. Form wiki: "The glass is coated with (or in some cases encases a layer of) a very thin almost transparent layer of metal (generally aluminium)"
that suggest it should have rotational symmetry - shouldn't create polarization.
Even it it would create some polarization, it should quickly equilibrate by bouncing with mirrors.

Observe that if such mirror would create even small difference of photon pressures/density, we could use this difference to create work from heat only - order energy which is stored in thermal fluctuations.

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I completely agree that we usually don't observe entropy reductions, but maybe it's because such reductions has usually extremely low efficiency, so they are usually just imperceptible, shadowed by general entropy increase... ?

2nd law is statistical mathematical property of model with assumed physics.
But it was proven for extremely simplified models!
And still for such simplified models was used approximation - while introducing functions like pressure, temperature we automatically forget about microscopic correlations - it's mean field approximation.
Maybe these ignored small scale interactions could be use to reduce entropy...
For example thermodynamics assumes that energy quickly equilibrate with environment ... but we have eg.ATP, which stores own energy in much more stable form then surrounding molecules, be converted into work...

5. Actually, a two-way mirror (AKA a one-way mirror, confusingly enough) is not perfectly transparent on one side and perfectly reflective on the other. In fact, I imagine it's a lot closer to 50/50. The trick is that they keep one side dark so there's less light to reflect from that side. You'll notice that the detectives are always standing in a dark room, while the suspect is always in a brightly lit room. Also, you'll notice that, from the transparent side, the mirror looks like tinted glass. That's because half or more of the light is being reflected back the other way.

6. Yes - I believe that it's not perfect - it's not making that all photons are on one side.

But it's not necessary - the slightest density gradient it would spontaneously create would make it Maxwell's demon - we could connect both parts to constantly equilibrate pressure of both parts.
Through this connection would dominate direction from higher to lower pressure, which we can use to create work (from heat) - for example placing there something like water wheel but made of mirrors.

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I would like to understand how two-way mirror works, could anybody correct me?

From wiki it is "The glass is coated with (or in some cases encases a layer of) a very thin almost transparent layer of metal (generally aluminium)." - it's asymmetric - metal from one side, glass from the other.
I've found information how to check if a mirror is two-way - when You place eg fingernail near normal mirror, there will be a gap between the finger and the reflection. But if Your finger directly touch the surface - it's two-way mirror.

So I imply that the reflective side is from metallic surface - looks intuitive.
I think that the effect of transparency from the second time is caused by superposition of many reflections thanks of partial internal reflection when light goes from glass back to let say vacuum - it's quantum effect.
?

7. BTW, a normal mirror is glass with one side coated with silver (which is what they also use on two-way mirrors IIRC), just a thicker layer.

Someone correct me if I'm wrong, but I don't think the asymmetry of the metalic film and glass is particularly important to the working of a two-way mirror. Here's an experiment you can try at home:
- Take an ordinary piece of glass. You'll have to work out where to get this, but a hobby store would be a good start.
- Carefully (use gloves, etc.) smoke up one side of the glass with a candle. Just a bit though, so you have slightly dark glass.
- Place the glass in something like a large cardboard box.
- From inside the cardboard box, where it's dark, you should be able to see out.
- From outside, where it's light, you should be able to see your reflection somewhat.

To verify what's important here, you can turn the glass around or try it without smoking it up. (Actually, you might want to try that first.)

8. I apologies for the two-way mirror example, I generally feel convinced now, that they work only because the difference in amount of light - the effect while looking on dark glasses could be explained for example by their curvature.
When I was thinking about it, I had a picture of destructive interference from anti-reflective coating.

But let's look at such coating...
http://en.wikipedia.org/wiki/Anti-reflective_coating
Let say: thick layer of higher refractive index material and thin of lower.
The destructive interference in thin layer happen only from anti-reflective side (thin layer) - shouldn't it reflect a bit smaller amount of photons than the second side? ... create gradient of pressure in photon containment - reducing entropy.

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