How Does a Two-Sided Mirror Work?

There are several uses for a two-sided mirror which include security and fun. Department store security and police often use them to watch suspects. Haunted houses use a two-way mirror to scare people. Others can use a two-way mirror for nefarious purposes like spying. This article will not only explain what a two-way mirror is and how they work, but also how to detect them.

Making a Mirror

When making a traditional mirror, a glass pane is used. This pane of glass has 2 different surfaces that are separate and both suitable for the silvering process (tinting). The surface on the outermost layer of the glass is the first surface. This will become the front of the mirror. The remaining layer is directly behind the thickness of the glass. In normal mirrors the reflective surface is applied to this layer. The reflective surface could be nickel, silver or tin. A layer of paint is often applied to further darken the back of the pane. What happens here is that all of the light is reflected toward the viewer. You cannot see through this mirror no matter how hard you try.

The Two-Way Mirror Process

Metal or reflective acrylic that is very thin is affixed to the glass pane's first surface. If you are looking in to a two-way mirror from this side you would see your image reflected back to you just like in a traditional mirror. This layer, considering that it is extremely thin, allows some light to come through the glass instead of simply being reflected back. This means that if someone is standing behind this mirror they can see through to the other side and the image would appear tinted. When these mirrors are installed between rooms, one side will appear to be a normal mirror while the other will simply look like a tinted mirror.

In order for this to work properly, the room where the mirror side is located has to be brightly lit. A common haunted house trick is to have someone in a bright room with the mirror and the actor on the other side in a dark room. When they want to scare you the light in your room is dimmed while their light is increased. This allows you to see them as a ghostly visage.

Detecting a Two-Way Mirror

There are 4 ways you can detect a two-way mirror:

  1. Fingernail Test: Touch the surface with your fingernail. In a normal mirror your finger and the reflection will not line up. If they do, then a two-way mirror is present.
  2. Hanging: Normal mirrors are simply hung on a wall and contain a frame. Two-way mirrors are often larger and are installed between rooms. If the mirror looks permanent then you most likely have a two-way mirror.
  3. Noise: Tap the mirror and if it sounds clear and sharp, then there most likely is another space behind it.
  4. Shade: Shade your eyes and press your face against the mirror and if you can see a space or defined shadows then it is a two-way mirror.