How Things Work: One-way mirrors act as useful surveillance tools

If you ever find yourself being interrogated in a room similar to one found in Law and Order, you might wonder if the mirror in the room is a one-way mirror, a mirror that is reflective on one side but transparent on the other. Once you know how one-way mirrors work, however, you can understand various ways to tell them apart from ordinary mirrors.

Let’s first look at how normal reflective mirrors work. Basically, a reflective mirror is a glass sheet with its back surface silvered. The silvered surface reflects the light that falls on the mirror. This reflected light creates the perception of an image behind the mirror.

One-way mirrors, on the other hand, have only their front surface partially silvered. This coating is very thin and allows a large amount of the light falling on the mirror to pass through, while reflecting the rest. The reflected light gives the perception of an image. The rest of the light passes through to the other side.

But if one-way mirrors let light through and the persons behind it can see you, why can’t you see them? The lights in the place behind the mirror are dimmed or the room is simply left dark; in contrast, the room in which the mirror is kept is brightly lit. So for the person looking into the mirror, a lot of the light falling on it from his or her side is reflected back, but barely any light is transmitted from the other side. However, a great deal of light passes through the glass into the dark chamber on the other side, allowing the people there to observe the activity in front of the mirror.

This knowledge leads to the critical question, “How do you tell if a mirror is a one-way mirror?” While there are multiple ways to tell them apart, one simple test is extremely effective. Extend your hand toward the mirror, so that your fingernail just touches the mirror. If you see a small space between your nail and its image, it is an ordinary mirror. However, if the nail and its image are touching each other, you should be aware that it is likely to be a one-way mirror and someone may be observing you. This technique works because ordinary mirrors have the back surface silvered, leading to a gap between the nail and its image. One-way mirrors, on the other hand, have their front surface silvered, so there is no gap.

Another technique involves turning the light off on your side and shining a flashlight at the mirror. This would reverse the effect of the one-way mirror, and you should be able to see through to the other side. In an ordinary mirror, you would just see your reflection.

Alternatively, you could rap on the mirror with a knuckle. A real mirror, which is backed by a wall, will give a muffled sound, while a one-way mirror will produce a much sharper sound. This sharp sound is similar to what you hear if you rap your knuckle against a pane of glass, such as a window pane.