How Does a One-Way Mirror Work?

One-Way MirrorThe one-way mirror. From one side, a mirror, from the other a window. It’s not the stuff of witchcraft, or even advanced optical technology – it’s really very simple:

Think of a mirror. A mirror is simply a very shiny surface. Light hits the object in front of the mirror, e.g. your face, then hits the shiny mirror surface, bounces back off of it and into your eye. Your brain recognises the light input as your beautiful reflection.

Now think of a window. Light can travel right through glass without a problem. Light hits the object on one side of the window, e.g. an animal in the garden, travels through the transparent window and into your eye. Your brain recognises the light input as the escaped lion from last night’s news.

A one-way mirror is somewhere between a mirror and a window – it has a very thin layer of reflective material, usually aluminium metal. The layer is so thin that some light can actually pass through instead of bouncing back.

With a one-way mirror some light is reflected and some light passes through, unlike a mirror or window where it’s all or nothing. For the effect we know and love from movie interrogation chambers, we need one final condition. One room must be brightly lit, the other very dark.

There is a lot of light in the bright room, meaning a lot of light passes through the one-way mirror to the dark room, and lot is reflected back. There is very little light in the dark room, meaning very little passes through to the bright room and very little is reflected. Basically it’s a bit like standing in a room at night time with the lights on looking outside: you can see your reflection clearly but can’t see much outside.

For the person on the bright side (the prisoner) the reflected light is far brighter than any light passing through from the dark side, meaning they can only see themselves. For someone on the dark side (the interrogators), the light passing through the window is much brighter than their reflection, meaning they see what is on the other side, and not themselves.

So if ever you are in a situation where you suspect you are being spied on through a one-way mirror, simply turn out the lights!

By Nick Waszkowycz

Picture credit: Camil Tulcan on Flickr

Article by Nick Waszkowycz

May 15, 2014

Nick studied Chemistry at university but decided that the pen was mightier than the conical flask. He decided to set off in search of a way to make his fortune from writing. He is still looking. But like all young men, Nick enjoys football, theatre and debunking conspiracy theorists. He shares his adventure in Berlin at and writes nonsense about football at Follow him on twitter at @nwaszkowycz.

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