Law of Reflection
Light, Sound and Waves

# Pepper's ghost

Classroom Activity for 11-14

In this activity, students build a modern version of a 19th century theatre illusion used by John Henry Pepper for creating ghostly characters on stage.

## Learning outcome

Students can provide an explanation of the Pepper's ghost illusion using the law of reflection.

## Preparation

This illusion is best performed in a dark room with blackout blinds. For a dramatic introduction, build your own and demonstrate it at the start of the lesson.

## Equipment

Each group of students will need:

• A moving image of a white ghost on a black background downloaded to a smartphone (eg search for “flying ghost gif”)
• A transparent dome with a diameter similar to the width of the smartphone (a domed plastic lid of the type used for iced tea or milkshakes is ideal)
• A flat transparent piece of plastic (eg a transparency film of the type used for overhead projectors)
• Marker pen
• Scissors
• Clear sticky tape

## Procedure

1. Place the dome on the flat plastic and draw around it using the marker pen.
2. Cut out the shape you have drawn. This will be your secret transparent screen.
3. Insert the transparent screen into the lid and trim as needed until it rests in the dome at a 45-degree angle. Fix in place discreetly using small tabs of clear sticky tape.
4. Open the moving ghost image full-screen on your smartphone.
5. Decide where the person viewing the trick (the audience) will sit.
6. Place your smartphone on the table screen-up so that the head of the ghost is pointing towards the audience.
7. Place the dome on the smartphone. Rotate it until the transparent screen is leaning towards the audience so that they can see an image of the ghost.

## Discussion prompts

• If the angle of incidence is 45°, what is the angle of reflection?
• Which direction is the light travelling in after it has reflected?

## Teaching notes

For a quick explanation, provide a simple ray diagram showing light incident on the transparent screen at an angle of 45°.

The angle of incidence equals the angle of reflection. Overall the light changes direction by 90°. The audience see a (virtual) image of a ghost because to them it seems like the light came from behind the screen.

For a more detailed discussion about image formation draw multiple rays from the head and tail of the ghost. Download the ray diagram below.

### Download

###### Law of Reflection
is expressed by the relation θ_i=θ_r
formalises Reflection