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Pepper's Ghost

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In 1862, John Henry Pepper, a chemistry professor at London Polytechnic Institute, began to redesign a concept developed by engineer Henry Dirck for presenting a ghost on stage. Pepper used the work of Dirck as a foundation to create a machine that used mirrors and lenses to project a ghostly image.  This concept is best explained by what happens when one is looking out the window of a dimly lit room when it is dark outside.  When the lighting is properly balanced, one will see a ghostlike image of themselves over the scene of the outside landscape.

 Around 1860, Dirck created a small working model of the effect. However, the design of the model was not sensible for a full size stage illusion. Professor John Pepper thought he had the solution to Dirck's problem. The two men became partners and began work together to bring the effect to the stage.  The men used a large plate glass pane, set at a 45 degree angle that reflected brightly lit objects off stage.  By doing so, the object off stage was shown on the stage. This picture shows the effect from below and atop the stage itself.   




In this case the ghost is an actor located forward and below the stage.  The glass pane pictures the reflection of the off stage ghost, while the ghost on the right shows what the audience thinks they see.


Lenses were later added to the effect to improve the clarity of the image.  Another later patent featured a large sheet of glass that was clear on one side and mirrored on the other.  This allowed the ghost to slowly fade from transparent to reflective. Using this concept, objects could easily fade in or out of a scene without the need to adjust intensity of lights.  The objects could also morph from one thing to another by sliding the glass side to side.



Although the effect permitted actors to interact with transparent objects, the illusion had limited usage for the stage.  The extremely large glass pane was not only difficult to place in position, but it was also very dangerous. In addition, the glass acted as a sound barrier between the performers and the audience, which presented a problem in a time without microphones.  The effect's debut was in a show, “The Knight Watching His Armor,” at the London Polytechnic Institution.  Pepper played a professor in this piece. 

In the 1860’s, the illusion appeared in public demonstrations all over the world.  Since then, the illusion concept of Pepper’s ghost has been used in several ways.  One practical use is in fighter planes.  In the plane’s head’s up display, a ghostly image of graphic flight and radar data float in front of the pilot.  When the effect was first invented, it was used in plays such as William Shakespeare’s Macbeth and Hamlet, and Charles Dickens’s A Christmas Carol.  Walt Disney World in Orlando, Florida contains a spectacular and most likely the largest example of the illusion of Pepper’s ghost.  The Haunted Mansion ride includes a section where the riders pass in front of a wall of mirrors and see a ghost sitting with them!  Even with all the strides that technology has made since the 1860’s, the apparatus of Pepper’s ghost is still being used today.   



The Haunted Mansion at Walt Disney World is perhaps the best and largest example of the effect known as Pepper’s Ghost.



Have you ever wondered why you can see your own face in the night passing in front of a dark store window in a lit street, but you won’t see it in the daylight?  Pepper’s Ghost demonstrates this phenomenon.  The illusion of Pepper’s Ghost seems to appear quite complicated, yet it is actually rather simplistic and with careful control of lightning, the effect can be truly startling and mystifying.  The only real device in Pepper’s Ghost is a single piece of glass, and that does not even need to be physically in motion.  The ‘Dircksian version’ is best demonstrated by looking out the window of a dimly-lit room during late dusk.  With properly balanced lighting, you see a ghostly image of yourself or an object, superimposed over the landscape outside.

The illusion Pepper’s Ghost deals with the branch of physics called optics.  Considered an optical illusion, Pepper’s Ghost is plainly a misinterpretation of an object that is present to one of more of the senses. The area of optics involves the nature and behavior of light, usually divided into two groups – geometrical optics and physical optics.  Geometrical optics deals primarily with the behavior of light rays and the formation of images and lenses, prisms, and mirrors, or in our case – glass.  It is thus very important in the design and construction of telescopes, microscopes, and other optical instruments.  In geometrical optics it is assumed that in a given uniform medium, light travels in straight lines, or rays.  Typical problems involve reflection of light rays by surfaces and the refraction of rays passing from one medium into another.  On the contrary, physical optics is concerned with the relationship between the properties of light and its nature.  The term ‘optics’ is often used to include studies of infrared and ultraviolet radiation, which are electromagnetic waves that are just outside the range of visible radiation. 

Since Pepper’s Ghost relies heavily on light, one must explore its importance.  Light is known to behave in a very predictable manner.  If a ray of light could be observed approaching and reflecting off of a flat mirror, then the behavior of the light as it reflects would follow a predictable law known as the Law of Reflection.  Initially, reflection is the turning back of waves at the boundary of a material.  The reflection of light waves is the most familiar example of reflection.  When light waves traveling in the air strike a material in which they have a different velocity, some light is reflected and some is refracted.  The amount of light that is refracted depends on the characteristics of the atoms and molecules of the reflecting substance.  Good reflectors, such as highly polished aluminum, silver, and steel, can reflect almost all the light incident on them.  When the eye receives reflected light, it traces the reflected rays back to the point from which the reflecting surface seems to make the rays depart.  This is how images are formed in a mirror, or in our case, glass.  All types of waves can be reflected.  Thus, light and other electromagnetic waves, sound waves, water waves, and the waves in a string all show reflection effects.

The Law of Reflection shows the determined path of a reflected wave.  The diagram below illustrates the Law of Reflection:



In the diagram, the ray of light approaching the mirror is known as the incident ray (labeled I in the diagram). The ray of light which leaves the mirror is known as the reflected ray (labeled R in the diagram). At the point of incidence where the ray strikes the mirror, a line can be drawn perpendicular to the surface of the mirror; this line is known as a normal line (labeled N in the diagram). The normal line divides the angle between the incident ray and the reflected ray into two equal angles. The angle between the incident ray and the normal is known as the angle of incidence. The angle between the reflected ray and the normal is known as the angle of reflection. (These two angles are labeled with the Greek letter "theta/θ” accompanied by a subscript; read as "theta-i" for angle of incidence and "theta-r" for angle of reflection.) The law of reflection states that when a ray of light reflects off a surface, the angle of incidence is equal to the angle of reflection.  As you see the image, light travels to your eye along the path shown in the diagram below:

It just so happens that the light which travels along the line of sight to your eye follows the Law of Reflection.  If one was to view along a line at a different location than the image location, it would be impossible for a ray of light to come from the object, reflect off the mirror according to the Law of Reflection, and next travel to your eye.  Only when you sight at the image, does the light from the object reflect off the mirror in accordance to the Law of Reflection and travel to your eye.  This is depicted in the diagram below:

 For example, in Diagram A above, the eye is sighting along a line at a position above the actual image location.  For light from the object to reflect off the mirror and travel to the eye, the light would have to reflect in such a way that the angle of incidence is less than the angle of reflection.  In Diagram B above, the eye is sighting along a line at a position below the actual image location.  In this case, for light from the object to reflect off the mirror and travel to the eye, the light would have to reflect in such a way that the angle of incidence is more than the angle of reflection.  Neither of these cases would follow the Law of Reflection.  In fact, in each case, the image is not seen when sighting along the indicated line of sight.  It is because of the Law of Reflection that an eye must sight at the image location in order to see the image of an object in a mirror.                               

For our Exploratorium we are creating a form of Pepper’s Ghost, yet without the extreme complication of the stage illusion dating back in the 19th Century. We used simple materials like a black box, Plexiglas, a glass of water, and a burning candle.  Just as stated before, the Physics branch of Optics plays a key role in this illusion.  When light travels from one medium to a different medium, part of the light is transmitted and part is reflected at the edge of the Plexiglas, or the interface.  The relative amount of transmission and reflection depends on the ratio of the refractive indices of the two media as well as the angle of incidence to the interface.  For normal incidence when light travels from air to glass, about 4% of the incident light is reflected and 96% is transmitted at one interface.  When the candle hits the Plexiglas, about 8% is reflected back to our eyes and 92% is transmitted through the Plexiglas, which we do not see because we are not inside the box to see it (there are two interfaces involved, another 4% is reflected when the light re-emerges from the glass to the air at the back surface of the glass).  There is no light sent out from behind the Plexiglas since the box is painted black.  It is important to note the black color of our box.  If the box were white, than 96% of the white would be transmitted through the Plexiglas and some of it would reach our eyes.  This transmitted light would be in competition with the weak reflected candle light.  In this case, the reflected light is too faint to see.  Therefore, the light that can reach our eyes is only the reflected candle light. 

In this figure you can see that a virtual image is formed behind the plane Plexiglas at the same distance from the Plexiglas as the object.  The size of the image is the same as the object.  When you catch the virtual image in a glass filled with water, it looks like the candle is lit in the water.


Work’s Cited

 “Adventures in Cybersound”:  6 pp.  Online.  Internet.  01 Jan 2003.

Davis, Rick.  “Pepper’s Ghost”:  5 pp.  Online.  Internet.  13 Dec 2002.

“Magic Kingdom Attractions”:  n.page.  Online.  Internet.  02 Jan 2003.

Mysteries of the Unknown: Phantom Encounters.  New Jersey:  Time-Life Books, 1988.

“Pepper’s Ghost”: 5 pp.  Online.  Internet.  13 Dec 2002.

The Physics Classroom. “Lesson 1: Reflection and Its Importance”: 4 pp.  Online.  Internet.          30 Dec 2002.

Robinson, Elsa E.  Merit Students Encyclopedia, Volume 9.  USA: Crowell-Collier Educational       Corporation, 1967.

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