The Illusion
The phrase “Pepper’s Ghost” might conjure images of a folk tale or scary campfire story, but it refers to an optical illusion created by scientist John Henry Pepper. The illusion, created for the stage, utilizes some really interesting physics to create an apparition like effect that mimics a ghost. This optical illusion is a fun way to connect physics to Halloween, and get your students wondering about other real-world applications for concepts learned in the classroom.
The technique utilizes two areas, one which the viewer sees, and another that is hidden from view. A glass pane is placed between the rooms at an angle that reflects the view of the hidden room to the audience, usually at a 45 degree angle. When a figure or object in the hidden room is lit, it will reflect on the pane, creating the illusion of an apparition in the other room.
The effect is popular in theme parks and museums, perhaps most famously being used in Walt Disney Parks for the Haunted Mansion and Phantom Manor attractions.These are intricate, big budget affairs, but the illusion can be recreated using a smart phone, some plastic, and a drinking glass or other glass container such as a cloche.
The Demo
Select your image or video on your smartphone or tablet, select something with a dark background and clear subject. Place the decive screen up on a table with the brightness all the way up.
Place your plastic piece inside the glass as close to a 45 degree angle as you can.
When you place the glass over the image on your device, you should see the image appear in the glass.
Another version of this demo can be done with the 3D Mirascope Illusion Maker, which utilizes parabolic mirrors to create a holographic image.
The Science
There are two types of images: real and virtual. A real image is one in which light rays actually come from the image. In a virtual image, they appear to come from the reflected image - but do not. In a flat mirror, for example, the virtual image of an object is behind, or "inside" the mirror, but light rays do not emanate from there. Real images form outside the system, where emerging light rays cross and are "caught".
This demo is a great introduction to these optics concepts, doing this demo around Halloween can be a really fun connection that engages students from the start!
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