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.
One way mirrors (their proper name since one side can see through the mirror but the other cannot) are basically semi transparent mirrors. What this means is that they reflect some light but they also let some pass through. This means that when placed between two rooms (as it is often used) it can allow one room to look through the mirror to the other, but to the opposite room it just looks like a mirror. This is achieved by having different light levels in each room, with the dark room being able to look into the brightly lit room. When the room has many bright sources of light in it, a lot of that light is reflected back to them, so the glass acts more as a mirror. It’s still see through but for the people inside the brightly lit room their eyes are adjusting to the bright light and most of what they see is their reflection, it’s very hard to discern detail behind the glass, though in certain cases it’s possible to see any small lights through it or some detail if you come really close to it. For the people in the dark room there’s very little light from their room to be reflected by the glass, but they can see the other room since all the light sources are from the other side. This means that the glass itself doesn’t only work one way, but it changes based on which side is more brightly lit.
The main distinguishing component of this is the fact that the material added to the glass (usually aluminum) is reflective. However the same principle applies to most types of glass, like tinted windows. Tinted windows are meant to block some of the incoming light, without necessarily using a reflective material to do so. So if we take for example the case of cars with tinted windows, looking from the outside into the dark interior you usually can’t see anything, the window, instead of reflective, simply appears black, but those on the inside can see inside because of the light level discrepancy.
You can even see the same effect with regular windows and glass. Since very clear glass is very expensive, most glass may appear completely transparent but it still reflects a small percentage of light back. I think you’ve probably noticed while looking through some windows that while you can see through them, you can also see your reflection on them. It all relies on the same principle and has to do with the differing light levels on each side of the glass.
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