Principles of Reflection and Optical Illusions

Introduction

Ghosts float and dance, a woman turns into a gorilla and a creepy organist disappears and reappears at will. How are these illusions created? Demonstrate the principles behind the classic optical illusion known as Pepper’s Ghost and discuss principles of optics and reflection.

Concepts

• Law of reflection
• Optical illusions
• Specular reflection

Materials

Safety Precautions

The materials in this demonstration are considered safe. Follow all laboratory safety guidelines.

Disposal

Store the apparatus for future use.

Procedure

1. The apparatus consists of two cubical boxes, one that is light-colored inside and out and one that is black. The white box has a “floor” extending out from the bottom, while the black box has a “roof” or overhang extension. The black box also has a hole cut in the top with a cover. Each box has a groove cut in the bottom or top extension, respectively.
2. Place the two boxes at right angles to each other to form an L-shaped apparatus (see Figure 1).
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3. Insert the acrylic glass plate into the grooved slots cut into the bottom and top extensions.
4. Place an object inside the black box, underneath the hole cut in the top. Place the cover on top of the hole.
5. Set up the demonstration apparatus for students to view so that students will be looking through the glass into the lightcolored box. They should not see what is inside the black box.
6. With the lights on, ask students what they see inside the light-colored box.
7. Dim the lights and narrate a spooky story, as desired, to “set the stage” for the optical illusion.
8. Remove the cover and shine a flashlight or other light source through the hole into the black box. Have students record observations.
9. Turn the lights on and off again and ask students to record observations and make predictions about what’s inside the box.
10. Repeat the demonstration and discuss the principles of reflection.

Teacher Tips

• Suitable objects include large, rotating, 3-dimensional ornaments, as well as toy figurines and crystals or rocks. Objects that themselves reflect light give shimmering images—these look very ghostly!

Discussion

In this demonstration, the light from a real object is reflected from a glass plate to create the illusionary object, which appears to be a part of the scenery behind it, and even appears to interact with it. This illusion is three-dimensional, detailed and as translucent as the operator desires, giving it a very haunted look. It can be startlingly life-like, but is very simple in its setup, relying only on reflected light. It is often seen in carnivals, haunted houses, and even the Haunted Mansion rides at the Disney^® Theme Parks. Many people mistake the illusion for holograms, but holograms cannot produce the detail and refinement of these images without advanced equipment. This demonstration requires only a mirror, a light source and a stage.

The optical illusion, Pepper’s Ghost, bears its name from John Henry Pepper (1821–1900), a chemist at the London Royal Polytechnic Institute. He did not invent the concept but made it possible for an existing concept to be stage-friendly, gaining widespread popularity and notoriety. The idea itself came from Henry Dirks (1806–1873), a Liverpool engineer. He developed the model that ultimately inspired Pepper.

The reflection from a mirror is very familiar. Mirrors are used nearly every day by people when they comb their hair or drive a car. Light is partially absorbed and partially reflected off every surface. The manner and amount of light that is absorbed or reflected depends on the type of material and the smoothness of the surface. Mirrors will reflect almost all light. Glass, though transparent, also reflects some amount of light.

Reflected light is governed by one simple principle: the angle of the incident light is equal to the angle of the reflected light, with respect to the normal line at the surface. This is known as the law of reflection (see Figure 2). The law of reflection is easily observed when light reflects off a smooth surface, such as a mirror, a shiny table, or even water. This type of reflection is known as specular reflection. The idea that light must be reflected at the same angle as its incoming angle was first noticed by Hero (or Heron) of Alexandria (fl. AD 62). An incoming ray is reflected at the same angle as the outgoing ray. The law of reflection is commonly expressed by Equation 1

where θ_i is the angle of the incident light and θ_r is the angle of the reflected light. When looking at a reflection from a flat mirror, the reflected image appears to be behind the mirror. The image cannot be seen unless one looks into the mirror. This is known as a virtual image. For a flat mirror, the virtual image is upright, the same size, and is located at the same distance behind the mirror as the object is located in front of the mirror (see Figure 3).

However, the reflected image will be reversed. This is why you often see the word AMBULANCE written AMBULANCE on the front of an ambulance vehicle. When people look in their rearview mirrors, they can read the word ambulance correctly and respond accordingly.

In the optical illusion demonstration, a sheet of glass is angled at 45° (see Figure 4). The Object Room is blackened, ensuring no excess light from the object will reach the glass. Then, any type of object may be hidden in the Object Room and illuminated. The light from the object will be reflected off the glass plate. At the same time, the glass plate continues to transmit light from the Image Room behind it. Light coming from the two sources is melded together, appearing to be one and the same to the observer’s eye. Using this simple trick, objects can appear to morph into each other, and entire ghostly scenes can be created.

References

http://www.phantasmechanics.com/pepper.html
http://www.precinemahistory.net/1860.htm
http://users.telenet.be/thomasweynants/peppers-ghost.html
http://www.cs4fn.org/illusions/makepeppersghost.php