These two articles explain the Bloody Mary optical illusion very well:
1) http://io9.com/5906432/an-optical-illusion-that-explains-the-origins-of-imaginary-monsters ; and
2) http://mindhacks.com/2010/09/18/the-strange-face-in-the-mirror-illusion/ .
You know, you’re staring into a mirror, in low light, for for some (say 5-10) minutes. Many teenage girls have done this, alone or together. And soon they see very strange things happen in front of them, in the mirror. Their faces become warped or turn into something else, often a very spooky experience.
A quote from article #1,
The descriptions [of what they experienced/saw] differed greatly across individuals and included: (a) huge deformations of one’s own face (reported by 66% of the fifty participants); (b) a parent’s face with traits changed (18%), of whom 8% were still alive and 10% were deceased; (c) an unknown person (28%); (d) an archetypal face, such as that of an old woman, a child, or a portrait of an ancestor (28%); (e) an animal face such as that of a cat, pig, or lion (18%); (f ) fantastical and monstrous beings (48%). (END OF QUOTE)
So it’s not difficult to understand why people believe in ghosts.
But how do the scientists explain this sort of dissociative identity effect (a.k.a. the Troxler effect)?
This quote from article #2 explains what happens,
…the dramatic effects might be caused by a combination of basic visual distortions affecting the face-specific interpretation system.
The visual system starts to adapt after we receive the same information over time (this is why you can experience visual changes by staring at anything for a long time) but we also have a system that interprets faces very easily.
This is why we can ‘see’ faces in clouds, trees, or even from just two dots and a line. The brain is always ‘looking for faces’ […]
According to Caputo’s suggestion, the illusion might be caused by low level fluctuations in the stability of edges, shading and outlines affecting the perceived definition of the face, which gets over-interpreted as ‘someone else’ by the face recognition system. (END OF QUOTE)
Unfortunately people used to magical & religious woo-bullshit thinking don’t know this. So they believe they’ve had a glimpse of a spiritual world, inhabited by spiritual beings.