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[Home] [Illusion] [Periscope] [Magic Fluid ] [Pinhole Camera] [Pepper's Ghost][phenakistoscope]
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[Home] [Illusion] [Periscope] [Magic Fluid ] [Pinhole Camera] [Pepper's Ghost][phenakistoscope]
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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? And you won't see it in the day light? or in front of a bright store window?
The Pepper's Ghost demonstrates this phenomenon. In the late 19th century a London showman named Pepper used a stage with a transparent window at an angle to the audience to display ghostly reflections of off-stage actors. So actors on stage behind the transparent window could walk through ghostly images which could appear and disappear to produce unexpected effects.
David Wall has found a simple way to build Pepper's Ghost using simple materials like a black box and Plexiglas. You can be the magician too making virtual images appear and disappear. Here you can learn the physics behind it.
Material list:
One cardboard box with the top removed, say 12" by 9" by 15 ". The dimension can vary according to availability. Choose one face to be the window. Say the face with 12" by 9" dimension.
Several sheets of black construction paper or black spay paint.
Plexiglas about the same dimension as the window face, say 12" by 9" by 1/8" thick.(Thickness is not crucial)
Two cardboard strips for the window frames at least 9" long by 2" wide each.
Candle and candle stick
One cylindrical mailing tube with 3" diameter and 6" tall, wide enough to fit over the candlestick.
One drinking glass
To assemble:
Cut the front of the box to make a window, leaving one inch on the sides and the bottom as a window frame. The top part should be open.
Glue or staple two strips of the cardboard on the sides just behind the window frame. There should be a gap between the window frame and the cardboard strips to allow the Plexiglas to slide in and out.
Glue black construction paper to make the inside of the box black, or paint the inside black.
Cut a full length slot in the mailing tube. The slot should be wide enough to see the candle flame from a wide angle, about a quarter of the circumference of the tube. This is used to hide the candle and the candle stick when you do the show.
To do:
Insert the Plexiglas in the window frame so that the box now has a transparent Plexiglas front.
Light the candle in the candlestick. Put the candle in front of the Plexiglas window. Now you should see a virtual candle image in the box.
Fill the drinking glass with some water. Locate the candle image position and place the glass there.
Hide the candle in the mailing tube so that your audience won't be able to see the candle from the front.
Look at the assembly from the front—you should see the candle burning in the water!
Physics Explanation:
When light travels from one medium to a different medium, part of the light is transmitted and part is reflected at 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 light hits the Plexiglas, about 8% is reflected back (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.) 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 is no light transmitted from behind the Plexiglas since the box is painted black. 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.
Why is it important to paint the box in dark color? If it is white, then 96% of the white will be transmitted through the Plexiglas Some of it will reach our eyes. This transmitted light will be in competition with the weak reflected candle light. In this case the reflected light is too faint to see. This is also the reason why in the daylight you see the inside of the store through the store window from the street , while at night under street lighting you see your own image reflected from a store window of a dark store. Another example; trying to look out of a lighted room at night, you see your reflection in the window. To actually see out, you have to light up the outside and turn off inside lights.
Extension:
You can also use a piece of Plexiglas with a convex curvature.
In this case the image is going to be smaller than the object and closer to the Plexiglas than the object..
The distinction between a virtual image and a real image: In a real image the light rays actually converge to form an image like in the pinhole camera case. While in a virtual image, your eye extends the rays to form an image like in the case of the mirror. Your brain thinks that the light came from the image behind the mirror. In both figures the images are virtual.
