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How does a two-way mirror work?

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For all their mystery, two-way mirrors are actually very simple devices. In reality, any mirror can be made into a two-way mirror; it’s not the glass that makes a mirror two-way, it’s the film on top of the glass.

Two-way mirrors have a semi-transparent film (also called a half-silvered surface) deposited onto one side of the glass. This film reduces, but does not eliminate, the amount of light that can travel through the glass. Too much film, and all of the light is reflected, and you have a regular mirror. Most two-way mirrors have just the right amount of film so that half the light comes through and half the light is reflected.

The real trick, however, has to do with the lighting in two adjoining rooms that share the mirror. As you’ll notice on television shows that use two-way mirrors, one room is brightly lit and one room is dim. In the room with plenty of light (i.e. the room with the suspect), there is so much light that even though half of the light travels through the mirror, enough light is reflected to make the glass appear like a mirror. In the dark room (i.e. the observing room), however, there’s not enough reflected light to cause any serious reflection; the light that enters from the lighted room is much stronger and the observers see through the glass instead of seeing their reflection.

If both rooms had equal lighting, everyone would see the same thing; a dim reflection and a dim outline of the people on the opposite side of the window.

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

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