One-way mirror


A one-way mirror, also called two-way mirror[1] (or one-way glass, half-silvered mirror, and semi-transparent mirror), is a reciprocal mirror that appears reflective on one side and transparent at the other. The perception of one-way transmission is achieved when one side of the mirror is brightly lit and the other side is dark. This allows viewing from the darkened side but not vice versa.

One-way mirrors for upper-level observation deck viewing down into a classroom (University of Wisconsin-Eau Claire)


The first US patent for a one-way mirror appeared in 1903, then named a "transparent mirror".[2][3]

Principle of operationEdit

Principle of operation of a one-way mirror showing that both parties predominantly see the person on the bright side.
The optical properties of the mirror can be tuned by changing the thickness of the reflecting layer.

The glass is coated with, or has been encased within, a thin and almost-transparent layer of metal (window film usually containing aluminium). The result is a mirrored surface that reflects some light and is penetrated by the rest. Light always passes equally in both directions. However, when one side is brightly lit and the other kept dark, the darker side becomes difficult to see from the brightly lit side because it is masked by the much brighter reflection of the lit side.[4]


One-way glass (4) used in a teleprompter

A one-way mirror is typically used as an apparently normal mirror in a brightly lit room, with a much darker room on the other side. People on the brightly lit side see their own reflection—it looks like a normal mirror. People on the dark side see through it—it looks like a transparent window. The light from the bright room reflected from the mirror back into the room itself is much greater than the light transmitted from the dark room, overwhelming the small amount of light transmitted from the dark to the bright room; conversely, the light reflected back into the dark side is overwhelmed by the light transmitted from the bright side. This allows a viewer in the dark side to observe the bright room covertly.

When such mirrors are used for one-way observation, the viewing room is kept dark by a darkened curtain or a double door vestibule. These observation rooms have been used in:

Smaller versions are sometimes used in:

The same type of mirror, when used in an optical instrument, is called a beam splitter and works on the same principle as a pellicle mirror. A partially transparent mirror is also an integral part of the Fabry–Pérot interferometer.

See alsoEdit


  1. ^ "How to Spot a Two-Way Mirror". Snopes. Retrieved March 28, 2020.
  2. ^ "The History of Mirrors". Retrieved 2015-04-22.
  3. ^ US patent 720877, Emil Bloch, "one way mirror", published 1903-02-17, assigned to Emil Bloch 
  4. ^ Finio, Ben. "Arduino-controlled RGB LED Infinity Mirror". Instructables. Autodesk, Inc. Retrieved 2017-08-12.
  5. ^ "Tour the Big Brother house camera runs". 2013-09-28. Retrieved 2014-03-06.

External linksEdit

  •   Media related to One-way mirror at Wikimedia Commons