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Pepper’s Ghost: Hologram Illusion

Real holograms are 3D images created by interference of light beams. They should be able to be viewed from any angle and allow the viewer to see the object from multiple sides. Real holograms can be seen without the need for special lenses, screens or other props.

In this activity, students will construct a pyramid that creates a reflected image from a digital screen display. This is a modern take on an illusion first described in 1584 by Neapolitan scientist Giambattista della Porta and popularized for theatrical use by John Henry Pepper in the 1860s. When a real or recorded image is reflected in a transparent screen at a 45o angle, viewers see a reflected virtual image that seems to have depth and appear out of nowhere .

This illusion continues to be used in amusement parks, theatres and other venues.

 

Objectives

  • Describe the locations of the real image, virtual image and reflection of light occurring in this activity
    Compare this optical illusion to the light physics of an actual hologram
    Make observations and develop questions for further exploration

Materials

  • Pyramid Projection template
    1–4 sheets of overhead transparency sheets
    Scissors
    Transparent tape
    Tablet or smart phone, and compatible videos such as: http://bit.do/peppersghost

Key Questions

  • Identify the location of the ‘real’ image and the ‘virtual’ image in your display How is this illusion similar to a real hologram? How is it different?

What To Do

  1. Use the Pyramid Projection Template to trace four tetrahedron shapes onto your overhead transparency sheet
  2. Carefully cut out the four tetrahedron shapes
  3. Use transparent tape to attach the 4 sides together and make a pyramid
  4. If needed, trim the narrow (bottom) end of your pyramid so it sits upright on a flat surface
  5. Use your tablet or smart phone to play one of the videos at http://bit.do/peppersghost. Place your pyramid in the middle of your screen with the narrow end facing down.
  6. Dim the lights in the room and look through the side of the pyramid! What observations can you make?

HINT: This activity works best if using very carefully & evenly cut shapes for the pyramid construction. Encourage students to take their time.

Extensions

  • What is it about the projection pyramid that makes the illusion work? Try changing some variables (i.e. shape, number of sides, construction material, type of video…) and comparing results Draw a ray diagram showing how light is reflected from the screen to your eye. Conduct your own research to learn more about actual holograms. Investigate a different optical illusion. Demonstrate and explain it to your classmates. What does it teach us about our eyes and/or our brains? Create your own video that would be compatible with a Pepper’s Ghost illusion Build your own illusion using the Pepper’s Ghost concept This illusion is improperly described as being a ‘hologram’ by many online sources. What would you call it instead? List some other examples of inaccurate information you have encountered on the internet.

Other Resources

Science World at TELUS World of Science | Units | Mirrors 

Science World at TELUS World of Science | Units | Illusions