Try staring at the blue rubbish bag hanging in the doorway at the back of this atmospheric room. Wait for about 1 minute. Your eyes will then adjust and you will see something a bit scary.
Optical illusions such as this have always been popular. Traditionally they would have been performed on stage by magicians with hidden devices. They work through our expectation that we are going to see something shortly. The room sets the scene effectively and our sense of tension and perhaps fear is heightened, so that when the object appears in our sightline, we are momentarily startled.