Pepper's Ghost is an illusion that's been used for decades to fool audiences into believing that an apparition is hanging in the air in front of them. It takes advantage of physical properties of light; when you place a transparent sheet of glass or plastic at an angle to a viewer, an object outside their field of view can be reflected from the sheet of glass back to the viewer. It's basic physics: The angle of incidence is equal to the angle of reflection.
One of the more prominent uses of this technique can be found at the Haunted Mansion ride at Disneyland. You're riding in a car that moves along the top of a large dining hall. You see ghosts dancing, flying, sitting at the table, and playing the organ below. I mused over that illusion for a number of years as a kid, trying to figure out how they pulled it off. There were certain clues. For example, I wondered why you were riding at the top of the dining hall, as opposed to beside it, like most of the other parts of the ride. I decided that it must be because the ride's creators don't want you to see something that's underneath you; then, it occurred to me that the ghosts could be reflections from that hidden area. That was exactly how they did it.
Pepper's Ghost is a pretty cool trick, but it suffers from a couple big drawbacks. First, you need a relatively large piece of plastic or glass. Second, if you view the plastic/glass from a broad angle from the side, you may see nothing at all.
Want to add some really amazing effects to your Halloween display at a relatively modest cost?Fortunately, there's a better way. You will need:
2. Projector
3. Media player
4. Videos
The following video demonstrates how you can create really mind-blowing ghostly apparitions in your windows and doorways. Here's how it works. You take the mosquito netting, and stretch it over the inside frame of a window facing the front of your home. Make sure that you remove as many wrinkles as possible. Put the projector in the room, and start playing the video. You may have to fiddle around with the brightness and contrast settings on the projector. The goal is to get the projected image to cast onto the scrim material so that an external viewer thinks that they're watching something floating in your room.
You don't need to buy videos. You can make them, yourself. The best approach is to use a green screen, capture footage of yourself and your ghoulies in your best ghostly or zombie attire, and then edit the footage in your favorite video editor. Most video editors allow you to remove a green screen background, and replace it with a black background.
Here's some footage of my upstairs window with projected footage of flying ghosts. One of my neighbors and his son stood outside for 20 minutes trying to understand how I did this. The illusion is really quite remarkable, and it took little more than 30 minutes to set up.