In the 19th century, John Henry Pepper (1821–1900) devised a technique for making transparent ghosts appear in a haunted house. The technique was named “Pepper’s Ghost” (pronounced “pepperghost”), after its inventor. The principle is as convincing as it is simple.

Below is an image with accompanying text.

On the left, you see what the viewer sees: a semi-transparent blue ghost playing a church organ. The spectre on the right (vxf2xf2r the black background) is not visible to the viewer. What the viewer sees is nothing more than a reflection of the spectre on the right on a transparent glass wall— a blue line has been drawn around it in the image.

This wall is set at a slight angle so that it can catch the reflection of the light. If you stand in front of your window at home and shine a spotlight on your face, you’ll see the (semi-transparent) reflection of your face in the window.

In the image above, the viewer sees a combination of that reflected image—so you’re actually looking at a piece of glass—and a real church organ.

When you apply this principle in a dark setting and combine it with spooky music, the effect is very convincing. In practice, it’s being used more than ever. Just think of The Phantom Manor at the Disney parks, the Haunted Castle at Efteling, and so on.