# how do one way mirrors work?

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how do one way mirrors work?

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They don’t. What you’ve seen as a “one-way mirror” is just a piece of glass with much brighter light on one side than on the other, so that the reflected light drowns out the light from the other side of the glass.

It has little to do with the glass and all about how you light the scenes on both sides of it. In general any glass will pass through some light and reflect some light. You can get slightly different coatings to change how much is reflected and how much is passed through but you can not have it work differently in one direction then the other. The only way to prevent light from getting passed through from the inside to the outside is to prevent light from hitting the glass in the first place. And this is done by keeping this side of the glass in complete darkness and the other side brightly lit.

There are two things at play.

Firstly as others have pointed out the lighting is an important factor.

The second is the coating. A mirror reflects because the glass is coated with a thin metal layer. In a one way mirror there are gaps in this coating allowing the light to pass through. But on the other side you don’t want it reflective, you want the coating to be black. That way the only light you will see from the mirror is the light shining through the gaps, none of the light from your side will be reflected.

This means that even if the lighting is the same on both sides it is far easier to see through in one direction than the other. Combine this with the correct lighting and it works very well.

As others have said, it’s all about the lighting. But I like numbers, so…

Suppose you had a mirror that was 50% reflective and 50% transmissive. And suppose the bright room had a brightness of 100units, and the dark room only 40units. (Doesn’t really matter what these units are.) In the bright room, you’d see 50units of light reflected (50% of 100units). But you’d also see 20units coming through from the dark room (50% of 40units). So in total, you can see 70units in the mirror, but most of this is coming from the bright room. Meanwhile in the dark room, you’d *also* see 70units of light, with 50units of this coming from the right room.

By tweaking the brightness of the two rooms and how reflective or transmissive the glass is, it can be pretty close to a one-way mirror.