Film Theory: The Use of Mirrors in movies

How many times have we been watching a movie, well in this case a terror one, and we see the character entering the bathroom and going to brush his teeth. In the bathroom there is a mirror, that shows in the begining of the shot that no one is there and when the character lowers his head to take the water, so does the camera, making the mirror be not seen, after that when he goes up, the mirror is there again and on the reflection is the character, looking that behind him is a person, ghost,spirit, etc.
In my case, I don't like when directors do this shot, why? Well, because I get scared, which is the main idea of it. You can be almost certain that in most horror films, there will be at least one scene that uses this kind of shot/effect. This shot is very popular because of the surprise that the viewer has when the camera goes up and shows something scary in the mirror with the character having a terrified look in his/her face.

We cannot be certain of who started to use this shot, since it has been seen throughout the history of films, being most in horror ones. One thing that you can be certain is that you will see this shot when you least expect it, and only once in a GOOD horror movie, because the exagerated use can cause boredom in the viewers(because they already know about the trick)

Mirrors are also used in films as elements that connect our world with the spirit one, according to most horror films, the passing of people between them is seen when the humans can see the ghosts or spirits through mirrros, etc, because of the superstitions that there are about them.