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Welcome to Brainstuff from How StuffWorks.com, where smart happens.

Hi, I'm Marshall Brain with today's question.

How do one-way mirrors work? Just about everyone has seen a

television show or a movie in which a criminal suspect is questioned while detectives watch from

behind a one-way mirror. How does a piece of glass manage to look like a mirror on one side

while remaining clear on the other.

The secret is that it doesn't.

A one-way mirror has a reflective coating applied in a very thin, sparse layer, so thin that it's

called a half-silvered surface.

The name half-silvered comes from the fact that the reflective molecules coat the glass so sparsely

that only about half of the molecules needed to make the glass an opaque mirror are applied.

At the molecular level, there are reflective molecules speckled all over the glass in an even film,

but only half of the glass is covered.

The half silvered surface will reflect about half the light that strikes its surface while

letting the other half go straight through. It turns out that half silvered mirrors are also essential

to many types of lasers, as well as to teleprompters. See how lasers work and how

teleprompters work at HowStuffWorks.com for details. So why doesn't the criminal suspect see the

detectives in the next room? The answer lies in the lighting of the two rooms. The room in which the

glass looks like a mirror is kept

very brightly lit, so that there's plenty of light to reflect back from the mirror's surface.

The other room, in which the glass looks like a window, is kept dark, so there's very little light

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