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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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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