BrainStuff Classics: How Do One-Way Mirrors Work?

How can a single piece of glass look like a mirror from one side but a window from the other? Learn how materials technology makes it possible in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:02):
Welcome to brain Stuff from How Stuff Works. Hi, brain Stuff,
Lauren foke obam here, I've got a classic brain Stuff
episode for you today, performed by our previous host, Christian Sagar,
and written by me. In this one, I wanted to
find out how one way mirrors work. Christian will explain. Hey,

(00:25):
I'm Christian Sager. This is brain Stuff, and today we're
talking about one way mirrors a K A two way mirrors,
a K A half silvered mirrors, a K, transparent mirrors,
a K A security mirrors, a K surveillance mirrors, a
K A observatoor tron's I don't I don't know if
that last one is a real thing. You know those

(00:45):
things that you see in crime dramas when one cop
is interrogating a suspect while another one watches through a
window that appears from the suspects side to be a
mirror That is not magic. TV cops aren't wizards, and
transparent mirrors their science. A regular old mirror, the kind
hanging over your bathroom sink. That's a sheet of glass

(01:06):
holding up an extremely thin layer of reflective metal. And
I mean thin, It's less than a micron thin. The
metal comes in the form of a metallic salt, which
can be dissolved in liquid and sprayed onto the glass
in a process called silver ing. That's because silver in
the form of silver nitrate, was the first stuff used

(01:26):
for this process. These days, most mirrors are actually silvered
with aluminum, which is cheaper and sturdier, and once it dries,
the reflective layer is given an opaque backing to first
of all protect it from scratches and then to stop
any light that manages to dodge through the metal layer.
Without this backing, you'd be able to faintly see the

(01:50):
wall behind it. See. Photons are tricky, and mirrors they're
not perfect. But what if you purposely make a mirror imperfect.
Manufacturers of translucent mirrors spray and even thinner, more sparse
layer of silvering onto the glass, meaning it reflects less light.
For example, let's say half the light of an ordinary mirror,

(02:13):
and okay, a little light gets absorbed by the silver ring,
but the rest passes straight through the glass like it's
a window, which it is. A translucent mirror is just
a reflective window, and it's a window from both sides.
Depending on the manufacturing process, it might be slightly more
reflective from one side, but you can look through it

(02:36):
from either. So how come the suspect sees his reflection
but the cop sees the suspect. Well, it's just a
trick of the light. The observer's room is kept dark
while the observes room is lit up very bright, So
on the cop side, more light is coming through the
glass than being reflected from the room, and from the

(02:56):
suspects side, more light is reflecting from the room than
being transmitted through the glass. And hey, people ask about
this a lot. If you ever want to test a
mirror to see if it's translucent, block the light around
you and try to peer through a bright flashlight can
help illuminate anything that might be behind that mirror. Today's

(03:25):
episode was produced by Tyler Clang. I'm realizing as I'm
going through these how many times I worked references to
wizards into my scripts. For a visual description of how
one way mirrors work, check out the video version of
this episode on YouTube, and of course, for lots more
completely translucent topics, visit our home planet, how stuff Works
dot com.