The Science Behind One-Way Mirrors: How Do They Work?

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A one-way mirror is a mirror that is partially reflective and partially transparent. The reflection is created by a thin coating of metal on the surface of the mirror. The amount of light that is reflected depends on the thickness of the metal coating. A one-way mirror can only be used if the lighting in the room on the reflective side is much dimmer than the lighting in the room on the transparent side.

In almost every cop movie, at some point or another, you will see a criminal being interrogated in a room, and a number of other officers will be carefully observing him from another room through a one-way mirror, also known, oddly enough, as a two-way mirror. Criminals can’t see the cops, but they can see the “perp” perfectly fine. Have you ever wondered how a mirror can only reflect on one side, but appear to be “see-through” on the other?


Regular Mirror Vs. One-way Mirror

A mirror is a smooth surface that is able to reflect almost all of the light that strikes it. The surface has to be smooth, as rough surfaces scatter light, rather than reflecting it.

Traditionally, mirrors are made of clear glass that has a coating of silver, aluminum, or tin on one side. This process is called silvering.  This ensures that all the light is reflected forward to the person standing in front of the mirror. Therefore, it is impossible to look through a regular mirror.

Cat looking at mirror
Credits:Anna Lurye/Shutterstock

In a one-way mirror, the reflective coating is made very thin, as compared to a regular mirror. The layer is so thin that only half of the light reflects back to the person standing in front of it, the rest goes through the pane, allowing those on the other side to see through.

Also Read: Why Do Mirrors Appear Silver?

The Importance Of Lighting

A genuine one-way mirror does not exist, as it would violate the laws of physics. Lighting plays a very important role for a one-way mirror to work properly.

Therefore, for a one-way mirror to work properly, the observer’s side where the glass looks like a window should be kept dark, so that very little light is transmitted through the glass into the interrogation room. The other room is lit up brightly, which ensures that there is plenty of light to reflect from the glass surface, so it looks like a regular mirror to the person standing in front of it.

If the lighting in both the rooms is equal, the glass will act like a tinted window, and people on either side will be able to see each other.

That’s why in the movies, you will always see the cops sitting in a dark room observing the bad guy through a one-way mirror, while the bad guy is in a brightly lit room and can only see his own reflection in the mirror.

If you ever suspect that someone else is watching you from the other side of a mirror, just switch off the light in your room and shine a bright flashlight against the mirror. That way, you’ll be able to see if there is someone on the other side.

Also Read: Why ‘Objects In The Mirror Are Closer Than They Appear’?

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References (click to expand)
  1. One-way mirror - Wikipedia. Wikipedia
  2. How do one-way mirrors work? - Science | HowStuffWorks. HowStuffWorks
  3. Michelle Bryner, (2012) How Does a One-way Mirror Work? - LiveScience
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About the Author

Ashwin Vinod has a B.Tech in Electronics and Communications from APJ Abdul Kalam Technological University, Trivandrum (India). He likes to watch movies, reading fiction novels and surf the internet.