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Speaker 1: Welcome to brain Stuff from How Stuff Works. Hi, brain Stuff,

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Speaker 1: Lauren foke obam here, I've got a classic brain Stuff

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Speaker 1: episode for you today, performed by our previous host, Christian Sagar,

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Speaker 1: and written by me. In this one, I wanted to

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Speaker 1: find out how one way mirrors work. Christian will explain. Hey,

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Speaker 1: I'm Christian Sager. This is brain Stuff, and today we're

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Speaker 1: talking about one way mirrors a K A two way mirrors,

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Speaker 1: a K A half silvered mirrors, a K, transparent mirrors,

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Speaker 1: a K A security mirrors, a K surveillance mirrors, a

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Speaker 1: K A observatoor tron's I don't I don't know if

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Speaker 1: that last one is a real thing. You know those

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Speaker 1: things that you see in crime dramas when one cop

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Speaker 1: is interrogating a suspect while another one watches through a

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Speaker 1: window that appears from the suspects side to be a

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Speaker 1: mirror That is not magic. TV cops aren't wizards, and

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Speaker 1: transparent mirrors their science. A regular old mirror, the kind

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Speaker 1: hanging over your bathroom sink. That's a sheet of glass

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Speaker 1: holding up an extremely thin layer of reflective metal. And

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Speaker 1: I mean thin, It's less than a micron thin. The

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Speaker 1: metal comes in the form of a metallic salt, which

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Speaker 1: can be dissolved in liquid and sprayed onto the glass

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Speaker 1: in a process called silver ing. That's because silver in

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Speaker 1: the form of silver nitrate, was the first stuff used

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Speaker 1: for this process. These days, most mirrors are actually silvered

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Speaker 1: with aluminum, which is cheaper and sturdier, and once it dries,

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Speaker 1: the reflective layer is given an opaque backing to first

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Speaker 1: of all protect it from scratches and then to stop

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Speaker 1: any light that manages to dodge through the metal layer.

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Speaker 1: Without this backing, you'd be able to faintly see the

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Speaker 1: wall behind it. See. Photons are tricky, and mirrors they're

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Speaker 1: not perfect. But what if you purposely make a mirror imperfect.

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Speaker 1: Manufacturers of translucent mirrors spray and even thinner, more sparse

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Speaker 1: layer of silvering onto the glass, meaning it reflects less light.

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Speaker 1: For example, let's say half the light of an ordinary mirror,

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Speaker 1: and okay, a little light gets absorbed by the silver ring,

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Speaker 1: but the rest passes straight through the glass like it's

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Speaker 1: a window, which it is. A translucent mirror is just

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Speaker 1: a reflective window, and it's a window from both sides.

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Speaker 1: Depending on the manufacturing process, it might be slightly more

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Speaker 1: reflective from one side, but you can look through it

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Speaker 1: from either. So how come the suspect sees his reflection

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Speaker 1: but the cop sees the suspect. Well, it's just a

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Speaker 1: trick of the light. The observer's room is kept dark

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Speaker 1: while the observes room is lit up very bright, So

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Speaker 1: on the cop side, more light is coming through the

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Speaker 1: glass than being reflected from the room, and from the

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Speaker 1: suspects side, more light is reflecting from the room than

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Speaker 1: being transmitted through the glass. And hey, people ask about

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Speaker 1: this a lot. If you ever want to test a

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Speaker 1: mirror to see if it's translucent, block the light around

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Speaker 1: you and try to peer through a bright flashlight can

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Speaker 1: help illuminate anything that might be behind that mirror. Today's

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Speaker 1: episode was produced by Tyler Clang. I'm realizing as I'm

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Speaker 1: going through these how many times I worked references to

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Speaker 1: wizards into my scripts. For a visual description of how

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Speaker 1: one way mirrors work, check out the video version of

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Speaker 1: this episode on YouTube, and of course, for lots more

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Speaker 1: completely translucent topics, visit our home planet, how stuff Works

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Speaker 1: dot com.