Just about everyone has seen a television show or movie in which a criminal suspect is questioned while detectives watch from behind a one-way mirror. How does a piece of glass manage to reflect light from one side while remaining clear on the other?
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The secret is that it doesn't. A one-way mirror has a reflective coating applied in a very thin, sparse layer -- so thin that it's called a half-silvered surface. The name half-silvered comes from the fact that the reflective molecules coat the glass so sparsely that only about half the molecules needed to make the glass an opaque mirror are applied. At the molecular level, there are reflective molecules speckled all over the glass in an even film but only half of the glass is covered. The half-silvered surface will reflect about half the light that strikes its surface, while letting the other half go straight through. It turns out that half-silvered mirrors are also essential to many types of lasers -- see How Lasers Work for details.
So why doesn't the "criminal suspect" see the detectives in the next room? The answer lies in the lighting of the two rooms. The room in which the glass looks like a mirror is kept very brightly lit, so that there is plenty of light to reflect back from the mirror's surface. The other room, in which the glass looks like a window, is kept dark, so there is very little light to transmit through the glass. On the criminal's side, the criminal sees his own reflection. On the detectives' side, the large amount of light coming from the criminal's side is what they see. In many ways, it's the same as if people were whispering in one room while a loud stereo played in the other. The sound of the whisper might carry into the room with the stereo, but it would be drowned out by the intensity of the music.
If the lights in the room with the mirror are suddenly turned out, or the lights in the observation room suddenly turned on, then the one-way mirror becomes a window, with people in each room able to see those in the other. You can see this effect in any mirrored office building at night -- if the light is on in an office, you can see into the office just fine.
Have you stood in front of a mirror and wondered if someone is spying on you?
In this article, we&#;ll show you several easy techniques for detecting a two way mirror.
A two way mirror is a transparent mirror that can look exactly like a regular mirror when the back side of it is dark. Don&#;t worry, there is more than one way to tell the difference!
Fingernail Test
Tint Comparison
Hollow Sound
We have a series of three tests to use to confirm that the mirror you see in front of you is not a two way mirror.
The glass two way mirror has a mirror coating that is 70% reflective. It only allows 11% of the light through the mirror. It is on a dark grey substrate, which makes it one-directional. The glass two-way mirror cannot cheat the fingernail test. The reflection coating must be facing the room that is being observed. That way the people being observed are only seeing their reflection. On the opposite side, the darker room is located where the observer will be watching.
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Notice the fingertip&#;s reflection touching itself. Very different from that of the standard mirror&#;s reflection.
The light is even on both sides of the mirror in this picture. You are able to see through the semi-transparent glass at the moment.
Front
Side
Back
Glass two-way mirror cannot cheat the fingernail test. The reflection coating must be facing the room that is being observed. That way the people being observed are only seeing their reflection. On the opposite side, the darker room is located where the observer will be watching.
70% reflective with 25% light transmission on a grey substrate, acrylic two way mirror is reflective from both sides of the substrate. Meaning it can be installed either way. If installed backwards ( the coated side on the back ) it can cheat the fingernail test looking like the reflection of a standard mirror! Something to think about if you are testing a questionable mirror. Acrylic tends to have a different feel than glass. Rubbing your nail gently across the surface may help you determine if you have found an acrylic two way mirror, or a standard glass mirror.
Acrylic two way mirror will closer match the reflection tint of a standard mirror as well. Whereas glass two way mirror tends to have a bronze tint.
Smart mirror is similar to the acrylic two way mirror in properties. It has a 70% reflection with a 25% light transmission. Its reflective from both sides of the substrate. Smart mirror can be installed backwards as well ( the coated side on the back ) it can cheat the fingernail test looking like the reflection of a standard mirror!
If you are testing a questionable mirror, smart mirror tends to have a warmer tone on the reflective coating. Observe the smart mirror on the right compared to the standard mirror reflection of the mug on the left.
Two-way mirrors are dependent upon the lighting in the room. Both sides of the mirror cannot be evenly lit for the illusion to work. The observer side of the mirror must be at least half as dark for the reflection coating on the other side to properly reflect light back at those being observed.
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