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? It's not magic, it's materials technology.

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:

SOURCES:

[justinlewtp]

I will never trust the mirror in the fitting room ever again

[alec]

I think I saw a comedy cop show where the suspects were in a lineup and this rookie cop turned on the light in the observing room and all the suspects saw the witness 😂 nice to see accurately applied science, specially in a comedy.

[fahadkhanye]

God this was killing me how they made these mirrors work. Great info thanks!

[beertjeswiss]

THX, everything i needed to know for my homework :P

[BabuForTheFriends]

If you wanna test if a mirror is trasparent you have to put your finger against the mirror: if you see a gap between you finger and its reflection it's a normal mirror, if your finger and its reflection touch it's a transparent mirror (i read this on the internet, if you have a transparent mirror try this so you can tell me if its true, for now on normal mirrors it corresponds to what the trick says)

[RosheenQuynh]

I really love Glasswing Butterflies and I think it'd be neat to have one crafted with a one-way effect but see-through on one side and the other has color :)

[DenorrisBanks]

this was a pretty good episode. you really drew me in. very entertaining and informative.

[Lerkero]

Great video, a lot of people don't understand how mirrors work and I think this explains it well, and simply enough.

[SelenaC_anime]

2:37

Well at least you guys didn't say "no space, leave the place."

[jasiahcisco]

Interesting. I was unaware of the process and wondered many times while watching shows and seeing them in person. Now I know! Thanks guys! Lol

[theknife]

I need one in my room if I’m gonna do some experiments on my friend

[gneu]

I like how whenever people hear of the term "being watched" they don't even think of a camera, they literally just think of a dude standing somewhere all day and observing you and doing literally nothing about it, only just observing.

[babbisp]

One-way mirrors are one of those things you'd expect to only see in sci-fi movies like invisibility or flying cars, but since they actually exist, everyone ignores how freakishly otherwordly they really are.

[oliviasonlinediary]

Thank you so much for this information! I understand a LOT more now :)

[AchillesBuena]

Me: *confused* maybe ill understand this when im older

2:10

Also ms: ohhh!

[_Bandit]

During the day, I can see outside through my window, but people outside can't see me, because inside my bedroom is much darker than there. At night, when I turn the lights on it's possible.

[djSpinege]

i always loved how in the Simpsons "Itchy & Scratchy focus group" you can hear the light switch being turned on. very accurate stuff.

[jasiahcisco]

You should do a video on how the internet actually works.

[Jameshg]

If you shine a light at a one way mirror does it go through or reflect, would it change if you shine it at different sides?

[Mj_]

Thanks a lot for this valuable information. Love from Bangladesh❤❤