A two way mirror or one way mirror is a type of mirror that is reflective on one side, but transparent on the other side. Here is an explanation of how two way mirrors work, their uses, and how to tell if a mirror is two way.
One Way or Two Way Mirror?
The original 1903 patent for this type of mirror named it a “transparent mirror.” Today, it’s called a two way mirror, one way mirror, reciprocal mirror, one-way glass, or semi-transparent mirror. It does not really matter what you call the mirror, unless you’re looking to buy one. Then, the term “two way mirror” appears preferable.
How a Two Way Mirror Works
A two way mirror is a sheet of transparent glass or plastic with a thin, nearly transparent metal coating. Light reflects off the metal coating just like it does in a normal mirror. The difference is that some of the light makes it through the glass or plastic. The mirror reflects on a brightly-lit side, but is transparent when viewed through a darkened side.
Buying a Two Way Mirror
The highest quality two way mirrors are glass. MirroPane and MirroView are two examples available at Amazon that differ by how much light they transmit/reflect and whether or not the glass is tinted. Glass two way mirrors are best for making smart mirrors. The disadvantages of glass two way mirrors are expense and risk of breakage.
Acrylic mirrors and mirror film are much more economical and are handy for making large two way mirrors. The main disadvantage is that the surface isn’t as flat as glass, so the two way mirror image will appear a bit wavy or distorted.
Two Way Mirror Uses
Two way mirrors have several practical and interesting applications:
- Mirrored sunglasses that reduce glare and ultraviolet reaching the eyes
- Mirrored windows that offer privacy and help keep buildings cool
- Smart mirrors that act as mirror that also show customized content
- Infinity mirrors for decorations
- Stage effects, such as Peppers ghost
- Security cameras
- Interrogation rooms
- Arcade video games
- Advertising wraps on windows
- Beam splitters in optics
How to Tell If a Mirror Is Two Way
In addition to legitimate two way mirror uses, some people use them for observing others without their permission. There are three clues that a mirror is a two way mirror:
- It’s built into the wall. Regular mirrors hang on walls or are attached to them, not usually built into them.
- You can see through the mirror to the other side. Cup your hands to minimize light and peer into the mirror. If it’s a two way mirror, you’ll be able to see through it. (Note: This is the best and most reliable method.)
- The fingernail test: Touch the mirror surface with your fingernail. If you nail touches the image, it may be a two way mirror. If there is a gap between your nail and the image, the mirror is silvered on the back and not the surface. It’s probably not a two way mirror.
Why the Fingernail Test Doesn’t Always Work
Usually, the surface of a two way mirror is the reflective surface. But, it doesn’t have to be that way.
Prove it to yourself: Closely examine a pair of mirrored sunglasses. When you wear them, you can see through them. But, a person viewing you cannot easily see your eyes. However, if you take the glasses off and hold them up to a light, you’ll notice you can see through the glasses from either direction.
As another example, make homemade silver holiday ornament using a chemical reaction to deposit silver inside a hollow glass or plastic ball. This is a simple two way mirror, but it won’t pass the fingernail test because the silver is inside the ball rather than on the exposed surface.
- Bloch, Emil (February 17, 1903). “Transparent Mirror”. US Patent 720877.
- Burdekin, Russell (2015). “Pepper’s Ghost at the Opera“. Theatre Notebook. 69(30: 152-164.
- Hopkins, Albert A. (1897) Magic, Stage Illusions, Special Effects and Trick Photography. New York: Dover Publications.
- Loudon, R. (2000). The Quantum Theory of Light (3rd ed.). New York: Oxford University Pres.