How do two-way mirrors work?

Eyes narrowed, the officer takes a swig of coffee as she watches the interrogation unfold in the next room. The suspect seems to be staring back, but all he can see is his own reflection, such is the magic of a two-way mirror. The secret lies in the design, which differs only slightly from the traditional kind.

Typically, mirrors are composed of a piece of glass covering a layer of metal (usually aluminium). When light passes through the glass and hits the metal it is reflected, which is why you see yourself when you look into it.

See how a two-way mirror works (first used in HIW Issue 79)

A two-way mirror also contains this metal coating, but much less of it is used. For example, if just half the mirror’s total surface area is covered by reflective molecules, the two-way mirror reflects only half of the light that hits it, meaning the remaining light can pass through to the other side. As long as the room on the other side is darkened, it will be possible to see through the mirror into the brighter room.

If you can’t see a gap when you put your finger against a mirror, it’s likely to be a two-way mirror

There is a way to check whether you’re facing a two-way mirror. Place your fingernail against the reflective surface, and if there is a gap between your fingernail and the reflected image, the mirror is genuine. If there’s no gap then beware, you could be being watched!

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