How can a single piece of glass look like a mirror from one side but a window from the other? It's not magic, it's materials technology.

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Hi! I’m Jonathan, this is BrainStuff, and today we’re talking one-way mirrors, aka two-way mirrors, aka half-silvered mirrors, aka transparent mirrors, aka security mirrors, aka surveillance mirrors, aka observer-ator-trons.

Y’know, those things you see in crime dramas when one cop’s interrogating a suspect, while another watches through a window that appears - from the suspect’s side - to be a mirror.

It’s not magic. TV cops aren’t wizards. Except sometimes they are. But that’s fiction, and transparent mirrors are science. Specifically, materials science and optics.

OK. A regular ol’ mirror – the kind hanging over your bathroom sink – is a sheet of glass holding up an extremely thin layer of reflective metal.

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 silvering. That’s because silver nitrate was the first stuff used for this process. These days, most mirrors are actually silvered with aluminum, which is cheaper and sturdier.

But silvering doesn’t make perfect mirrors: They reflect most light, but a little is still transmitted through infinitesimal gaps in the reflective metal layer. So everyday mirrors receive an opaque backing, like dark paint. This stops cold any photons that slip through the metal layer (and protects it from scratches). Without the backing, you’d be able to faintly see the wall behind the mirror.

But what if you purposefully make a mirror imperfect? Manufacturers of transparent mirrors spray an even thinner, less dense layer of silvering onto the glass.

Meaning it reflects less light – for example, let’s say half the light of an ordinary mirror. The rest passes straight through the glass like it’s a window. Which it is. A transparent mirror, with its sparse silvering and lack of backing, is just a reflective window.

And it’s a window from both sides! So: How come the suspect sees his reflection, but the cop sees the suspect?

It’s a trick of the light. The observer room is kept dark, while the observee’s room is lit up like the Vegas strip. So on the cop’s side, more light is coming through the glass than being reflected from the room. And from the suspect’s 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 transparent, block the light around you and try to peer through. A bright flashlight can help illuminate anything that might be behind the mirror.

CREDITS:
"Bean" movie clip from MovieClips.com YouTube channel:
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