One-way mirrors are synonymous with detective scenes. A suspect sits in a bright, white room with a big mirror on one of the walls, and on the other side cops sit in darkness to observe. That’s all well and good, but there are plenty of places where a one-way mirror would creep us out. How can you tell the difference?
One-way mirrors are basically just windows. You know how when you look out of a window in your house, you can see your own reflection overlaid with what you see outside? And when it’s really dark out, you can see even more of your reflection and less of what’s outside. If you were to go outside, though, you could easily see inside the house because the lights are on.
Bringing it back to the police station interrogation situation, it’s really bright in the room where the suspect is, and really dark in the room where the cops are viewing. Because of the stark contrast in lighting, the suspect will only be able to see his own reflection in the glass and nothing coming from the other side. While from the cops’ perspectives, they can easily see into the lit room just as if they were looking into a house at night from the outside.
Normal mirrors use a reflecting surface to bounce incoming light back and do not let any light pass through; this reflecting surface sits on a level below the top layer. Simple enough.
So let’s say you are in a dressing room and paranoia sets in that you are being watched from behind the mirror! Simply place the tip of your finger against the mirror and look at it from the side. With a true mirror, you will see a gap between your finger tip and its reflection. This is because of the gap between the top layer and reflecting layer in the mirror itself. However, if the tip of your finger is touching its reflection then you should probably pick a different dressing room (and alert the authorities).