How One-Way Mirrors Work

Today I found out how one-way mirrors work (also known as “two-sided” mirrors).

Interestingly, there is no such thing as a true one way mirror.  This would actually violate the laws of physics (for more on that see the Bonus Factoids below).  What’s actually going on here with one-way mirrors is that we have a piece of glass with an extremely thin coating of some reflective material like silver (as is often used on normal mirrors).  Unlike normal mirrors though, the silver layer is so thin that it only actually reflects about half the light back.  The other half passes through the glass.  This layer is actually so thin, that if you were to look at it at a molecular level, you’d see that only about half the glass was actually coated with the silver; the silver would appear to be speckled on the glass more or less evenly.

The problem with allowing half the light through though is that you would normally still be able to see either way through the glass; it would just appear tinted.  Not only that,  the half of the light being reflected is being reflected back on both sides.  So either side could be the reflective side or the “see through” side.  The variable here is only the light levels in the two rooms which determines which side seems see-through and which side seems reflective.

If the lighting in both rooms is close to equal, the strong mirroring effect won’t happen because the light being passed from one side to the other is roughly equal.  It will then just be very similar to a normal window, though with the windows seeming slightly tinted.  However, if you turn off the lights on the one side and make the lights on the other side very bright, then the mirroring effect will kick in quite strongly, effectively stopping someone from seeing what’s on the other side of the window.  What you’ve done here is create an imbalance of light traveling between the rooms.

If it seems confusing why creating this imbalance should make a difference, think about if you had the volume of your radio turned up really loud in one room and another radio set really softly in an adjacent room.  The person in the loud room would not be able to hear the quite radio in the adjacent room as it is drowned out by the loud radio.  If your volumes are set at roughly equal decibel levels, then you’ll both be able to hear each others radios.

You can see a similar effect even with a normal window in your house that contains no reflective coating.  When it is dark outside and bright in the room you are in, the window ends up showing a strong reflection on the light side while people on the dark side can see inside just fine.  The same type of thing is happening with one-way mirrors, except they’ve added a thin reflective coating in order to make the effect even stronger.

Of course, an easy way to get around this problem is to simply approach the window and block out the light on the bright side with your hands.  You’ll then be able to see through the window or one-way mirror just fine.

Bonus Facts:

  • A true one way mirror that allows light through one way, but no light through the other is not possible as it violates the second law of thermodynamics.  To illustrate why that would be, think about if you had a hot object on the blocking side and a cold object on the transmitting side.  In this case, radiant energy would then be traveling from cold to hot, while energy on the hot side would simply be reflected back into the hot side; so the net effect would be the heating of the hot side from the cold side without adding any energy to do so, thus violating the second law of thermodynamics.
  • The Italian town of Viganella gets no direct sunlight for about seven weeks each winter.  In order to solve this problem, in 2006, a computer controlled mirror was installed which is approximately 25 feet by 15 feet.  The mirror is controlled such  that it reflects sunlight into the town’s main city square during the day time.
  • Traditional mirrors are typically made with two distinct surfaces.  The outermost layer is just glass.  The inner layer is generally made of a very thing layer of nickel, silver, or tin.  The back is then usually painted black, to further darken the pane.  There is also often something like Tin applied between the silver and the glass as silver will not naturally bond to glass.  Another common additive to these two layers is copper, between the silver and the black paint.  This helps protect the silver in the mirror from getting damaged when handled.
  • Mirrors in ancient times were typically just small pools of collected water with the vessel containing the water having a dark color to it.
  • As early as 6000 BC in Turkey, people started manufacturing non-water mirrors.  These early mirrors were typically made of obsidian, which is a naturally occurring volcanic glass.  These pieces of obsidian would be cut appropriately and highly polished to give off a reflection.
  • Around 4000 BC, the first polished copper mirrors started showing up in Mesopotamia.  Around 2000 BC, the Chinese began manufacturing similar bronze mirrors.  These types of mirrors were only available to the extremely wealthy.
  • Fast forward to around the 16th century in Venice, and we see the first examples of glass mirrors with tin-mercury backings, which produced reflections very similar in quality to the mirrors we have today.  These mirrors were ridiculously expensive due to their high quality, expensive materials, and how fragile they were for the types of transport available at the time.  Eventually, the French succeeded in learning how the Venetian glass makers made the mirrors and succeeded in lowering the price quite a bit on mirrors in  Western Europe.  Although, the mercury in the mirrors, being toxic, was still a major problem and they were still too expensive for most people to afford.
  • The silvered-glass mirrors we use today were invented by German Chemist Justus von Liebig in 1835.  He was able to create an extremely thin layer of silver on glass through the chemical reduction of silver nitrate.  This process led to a much greater availability of affordable mirrors, thus effectively making available high quality mirrors to the masses for the first time in history.
  • It has only been as recent as 2001 that scientists have been checking to see if various animals could recognize themselves in mirrors.  Before then, it was thought that only humans had that kind of self awareness.  As it turns out though, it is still thought that most animals do not recognize that mirrors show reflections of themselves.  Animals that have been proven to be able to recognize themselves include Asian elephants, Apes, Pigs, Chimpanzees, Dolphins, Magpies, Orangutans, European Magpies, humans, and my cat Syrup (though not my cat Waffles).  Interestingly, this is not the first set of cat’s I’ve had where one clearly knows they are looking at themselves in the mirror and the other clearly does not realize this.  In both cases, it was the dumb one that couldn’t understand what they were seeing in the mirror.  Supposedly, cat’s and dogs cannot recognize they are seeing themselves, but I’m going to have to call BS on that one as far as applying it universally to all dogs and all cats; from my experience, some can and some can’t.  The “mirror test”, recently developed and used by scientists, is fairly widely thought to be flawed, producing a lot of false negatives, even among some of the groups that have been shown to be able to recognize themselves in mirrors on the individual level.  So it is a good test for proving that animals who pass the test can in fact recognize themselves in mirrors, but doesn’t necessarily say anything about those who don’t pass the test.  Considering the test used is flawed and the research done in this area is so new, it is entirely possible that quite a lot of animals are able to recognize themselves in mirrors; the jury is still out on that one.
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  • Very interesting, thank you. I’m afraid there is a small mistake in the text. Turkey didn’t exist in 6000 BC.

  • Second paragraph states “for more on that see the Bonus Factoids below”
    Below you have “Bonus Facts” not “Bonus Factoids”

    Third Bonus Fact “The inner layer is generally made of a very thing layer” looks like there is an extra “g” in there.

    As usual, an interesting and informative article.

  • Why is it called ‘one way mirror’? I think it’s confusing, at least to me. I would think that normal mirror is ‘one way’ in that it only works from one direction. Wouldn’t ‘one way glass’ or ‘two way mirror’ be more accurate?

    • no, because it is a mirror one way and a window the other way, therefore it is a one way mirror.

  • Can anyone tell me what happens if I place two one way mirrors, with the reflecting surfaces facing each other, and pass a light ray from outside. The system is not enclosed, lets assume we’re in open air. Sunlight has complete access.