How do one-way mirrors work?

One-way mirrors are ubiquitous in crime dramas, but how do they work? Discover the secret behind one-way mirrors in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:00):
Welcome to brain Stuff from how Stuff Works dot 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

(00:21):
criminal suspect is question 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

(00:43):
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 and opaque mirror are applied at the
molecular ball. There are reflective molecules speckled all over the

(01:03):
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 goes 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

(01:25):
how teleprompters work at how stuff works dot com for details.
So why doesn't the criminal suspects 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 surface.

(01:47):
The other room, in which the glass looks like a window,
is kept dark, so there's very little light to transmit
through the glass. On the criminal side. The criminal sees
his own reflection. On the detective side, the large amount
of light coming from the criminal side is what they see.
In many ways, it's the same as if people were

(02:07):
whispering in one room while a loud stereo were playing
in the other. The sound of the whisper might carry
into the room with the stereo, but it would be
drowned out by the intensity of the music. If the
lights in the room with the mirror are suddenly turned out,
or the lights in the observation room suddenly turned on,
then the one way mirror becomes more like a window,

(02:29):
with people in each room able to see those on
the other, or if you reverse the lighting, the mirror
side switches to the detective side and the criminal can
look through and see the detectives. Be sure to check
out our new video podcast, Stuff from the Future. Join
how Stuff Work staff as we explore them as promising

(02:50):
and perplexing possibilities of tomorrow. The House Stuff Works. I
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