One Way Mirrors

A one way mirror seems to be one of the best ways possible to spy on another human. They are almost impossible to recognize and also really easy to create. Most importantly, although everyone knows they exist, rarely do people actually suspect a mirror of being anything other than a simple mirror. Luckily one simple method to check if a certain mirror is a one way mirror would be the fingernail test. You just need press the tip of your finger against the mirror and if there's no gap (between your finger and your finger's reflectiom), it's definitely a one way mirror.

However only the most common type of two way mirrors can be caught using that method. You could also try knocking and trying to hear if there's a hollow space behind the mirror. Remember, there doesn't need to be a huge space back there, just enough to fit a small camera.

The test I would recommend would be the flashlight test (all somewhat newer phones have built in flashlights). Just press your flashlight (phone) all the way against the mirror and you'll definitely be able to tell if the light shines through. This is due to how one way mirrors are fundamentally built.

If you haven't noticed yet, you basically have to be able to directly touch the mirror in order to verify if it's real.

One way mirrors are sometimes also referred to as two way mirrors. Technically a one way mirror is made using a two way mirror placed between a dark room and a well lit room. A mirror is basically a layer of glass on top of a reflective metal sheet. If this metal sheet is thin enough only a portion of the light shone upon it will be reflected. With a bit of testing, you'll be able to achieve something that reflects about 90% of the light shone upon it. Thus you'll have a slightly translucent reflective piece of glass. If you placed this new mirror in the wall between two rooms, person A in room A would be able to see his own reflection but at the same time a rough image of the other room. Now if person B entered room B, he would be able to see person A at 10% intensity and himself at 90%. That's why it's known as a two way mirror.

If you turn off the light in room A, all of a sudden, the person in room A can perfectly see room B, although he has lost the ability to see his own reflection. Person B would just see his own reflection in a seemingly normal mirror. Although the reflection would be slightly weaker, at around 90%, you would never notice the difference.

Voila, you now know how unrealistic most one way movie mirrors are.