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How can you tell if a mirror is two-way?

Home » Gadgets » Two-Way Mirrors » How can you tell if a mirror is two-way?

One popular chain email warns that department stores and restaurants use two-way mirrors, and thus, employees can watch you change clothes and go to the bathroom. The e-mail suggests that if you put your fingernail up against a mirror and there is no gap between your fingernail and its reflection, then it’s a two-way mirror. (Some versions of the e-mail also suggest throwing a chair through the mirror).

In reality, however, the fingernail test only proves whether the mirror has an extra layer of glass. So-called first surface mirrors lack the extra layer of glass contained in second surface mirrors, and thus, first surface mirrors show no gap between an object and its reflection. Two-way mirrors, however, can be either first or second surface mirrors (most are second), so the fingernail test has nothing to do with whether someone can watch you from the other side of the mirror.

There are ways, however, to determine if a mirror is working both ways, and most of these methods depend not on the glass, but on the lighting in the room. The simplest method is to turn off the lights in the room. If the mirror is a two-way mirror, reducing the light in the room will eliminate the necessary light to create a reflection, and the mirror will act as window.

Alternatively, if you have a laser pointer or a flashlight, put it directly against the glass; if it is a two-way mirror, the light will go straight through the glass. If you don’t have a flashlight, however, cup your eyes and put them up against the glass; this act will artificially reduce the light and allow you to see through a two-way mirror.

Finally, you may be able to tell if a glass is a two-way mirror by simply tapping on it. A regular mirror is mounted on a wall or other backing, so you will hear a dull thud when you tap on it. A two-way mirror, however, always forms part of the wall itself (because it also serves as a window), so if you tap on it, it will sound hollow.

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