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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
The
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 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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