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Speaker 1: Welcome to brain Stuff from how Stuff Works dot com

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Speaker 1: where smart Happens. Hi, I'm Marshall Brain with today's question,

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Speaker 1: how do one way mirrors work? Just about everyone has

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Speaker 1: seen a television show or a movie in which a

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Speaker 1: criminal suspect is question while detectives watch from behind a

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Speaker 1: one way mirror. How does a piece of glass manage

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Speaker 1: to look like a mirror on one side while remaining

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Speaker 1: clear on the other. The secret is that it doesn't.

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Speaker 1: A one way mirror has a reflective coating applied in

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Speaker 1: a very thin, sparse layer, so thin that it's called

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Speaker 1: a half silvered surface. The name half silvered comes from

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Speaker 1: the fact that the reflective molecules coat the glass so

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Speaker 1: sparsely that only about half of the molecules needed to

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Speaker 1: make the glass and opaque mirror are applied at the

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Speaker 1: molecular ball. There are reflective molecules speckled all over the

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Speaker 1: glass in an even film, but only half of the

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Speaker 1: glass is covered. The half silvered surface will reflect about

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Speaker 1: half the light that strikes its surface, while letting the

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Speaker 1: other half goes straight through. It turns out that half

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Speaker 1: silvered mirrors are also essential to many types of lasers,

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Speaker 1: as well as to teleprompters. See how lasers work and

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Speaker 1: how teleprompters work at how stuff works dot com for details.

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Speaker 1: So why doesn't the criminal suspects see the detectives in

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Speaker 1: the next room. The answer lies in the lighting of

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Speaker 1: the two rooms. The room in which the glass looks

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Speaker 1: like a mirror is kept very brightly lit, so that

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Speaker 1: there's plenty of light to reflect back from the mirror surface.

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Speaker 1: The other room, in which the glass looks like a window,

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Speaker 1: is kept dark, so there's very little light to transmit

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Speaker 1: through the glass. On the criminal side. The criminal sees

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Speaker 1: his own reflection. On the detective side, the large amount

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Speaker 1: of light coming from the criminal side is what they see.

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Speaker 1: In many ways, it's the same as if people were

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Speaker 1: whispering in one room while a loud stereo were playing

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Speaker 1: in the other. The sound of the whisper might carry

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Speaker 1: into the room with the stereo, but it would be

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Speaker 1: drowned out by the intensity of the music. If the

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Speaker 1: lights in the room with the mirror are suddenly turned out,

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Speaker 1: or the lights in the observation room suddenly turned on,

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Speaker 1: then the one way mirror becomes more like a window,

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Speaker 1: with people in each room able to see those on

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Speaker 1: the other, or if you reverse the lighting, the mirror

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Speaker 1: side switches to the detective side and the criminal can

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Speaker 1: look through and see the detectives. Be sure to check

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Speaker 1: out our new video podcast, Stuff from the Future. Join

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Speaker 1: how Stuff Work staff as we explore them as promising

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