*2-way Security Mirror A two way mirror, a mirror that is transparent on one side and reflective on the other, is created by placing a layer of semi-transparent film of aluminum on gray-tinted glass. This allows a percentage of light to pass through and reflects the rest.
We have all seen a cop drama TV show or film in which a criminal suspect is interrogated while police watch from behind a mirror. Why can’t the suspect see those watching him behind the mirror? The answer lies in the level of lighting in both rooms. The room the suspect is in is kept very brightly lit, so that the light in the room reflects off the mirror’s surface. The observation room containing the police, on the other hand, is kept dark, so that little to no light gets through the glass. As a result, the suspect sees only his own reflection and the police see into the suspect’s room clearly. If the lights in the suspect’s room were suddenly turned out, or the lights in the observation room suddenly turned on, then the two way mirror would become a window. It’s the same sort of effect you see in a mirrored office building at night — if there is a light on in one of the offices, you can see into that office just fine.