The use of mirrors as a horror device is common. I recall the
submarine horror movie "Below" and the game "Silent Hill 3" both
use mirrors effectively to scare people.
I think the physics concept of a virtual image (as opposed to a
real image you can focus on a surface) is something very disturbing
to us human beings. Long before the invention of cyberspace, the
alternate reality of folklore resides in the other side of the
mirror.
