# how are 2 way mirrors are made?

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You know those windows in police rooms? Like one side you can see into the questioning room but in the questioning room it’s a mirror? How are those made? How do you see out of them?

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“2 way mirrors” do not exist. They would violate thermodynamics. What people call “two way mirrors” are just normal glass with a *lot* of glare, created by adding some reflective metal to one side.

This allows the darker room to see into the lighter room, while the lighter room mostly sees their own reflection.

You know how during the day you can’t see the stars because the sky is too bright? A two way mirror works on the same principle. It’s a piece of glass with a reflective coating on the backside, but the reflective coating doesn’t reflect *all* of the light. It lets some through. This way, if one of the rooms is dark and the other is bright, people in the bright room can only see their reflection because there’s so much more light reflecting than coming in from the dark room. People in the dark room can only see light from the bright room because their reflection is so dim.

Glass reflects about 5% of light. The one way mirror effect is an illusion where the room on the mirror side is so bright that the 5% that is reflected is brighter than the light passed through from the window side. The reflection washes out the view from the other side. For this to work, the room on the window side must be dark or much less dimly lit than the room on the mirror side.

It’s like at night in your home. The lights inside reflect your image or the interior of your house anyway. But people outside can see in. Same same just on purpose

They’re also called half-silvered mirrors. If you put a very thin layer of reflective material on a piece of glass, instead of the thicker layer you’d use to make a normal mirror, some light from each side can shine through and some of the light will bounce off.

If one side of a half-silvered mirror is a brightly lit room and the other is a dimly lit room, both sides will see an image composed of half the light striking the mirror from the bright side and half the light striking it from the dark side, i.e., an image composed of mostly light from the brightly lit room.

Mirrors are made by coating a piece of glass with silver or another reflective metal, usually aluminium.

Traditionally, silver was used because if you dipped glass into a solution of silver nitrate and sugar, the solution would leave a layer of silver on the glass. These days, aluminium is available and it can be evaporated onto glass by heating aluminium wire and the glass in a vacuum oven.

There is a certain thickness of metal needed to make the reflective layer completely reflective and get a good mirror. If you don’t put enough on, then the metal will be thin enough to see through although it will still be shiny and reflective. In the case of “two way” glass, this thin metal layer is made deliberately.