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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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Magic Fluid
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In this optical show you can be a magician. You can make the figure under the cup appears when you pour some "magic fluid".
Material list:
a) Any glass sphere of diameter 2.5 to 3.0 cm. I use a glass shooter marble about 2.5 cm in diameter, which can be purchased from any toy shop at about $2.00 for six.
b) 1 small plastic or glass cup that the glass marble can sit snugly at the bottom. If you find something large you can use Play Dough to make a ring for the glass ball, so that the glass marble won't roll around too much.
c) one cup of water. This is the magic liquid!
d) a piece of paper with some figure or letters on it which you want to use for the show.
To do:
Put the cup with the glass marble over the figure. Ask your audience to verify that he/she cannot see any figure above the glass sphere. See figures 1 and 2. Then pour your magic liquid slowly over the glass sphere; be sure that the glass sphere does not roll around too much and you do not move the paper underneath. Now a figure appears above the sphere! See figure 3 below
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How does it work?
When light passes from one medium to the other, its speed changes and the light ray bends. This is called refraction. When light passes from air to glass, light slows down and the light ray bends towards the normal of the refracting surface. When light passes from glass to air, light speeds up and bends away from the normal. In order to see an image, the light rays have to focus on your eye retina. When only the glass sphere is in the cup, the figure underneath is focused before your eye retina, so you see only some blurry figure like in figure 2. As you pour the water into the cup the light passes from water to glass then water before it reaches your eye. The light speed changes are smaller from water to glass than from air to glass, so that the light ray does not bend as much as from air to glass. Therefore the focal length of the assembly of water and glass is longer than that of air and glass. The figure now focuses on your retina, so that you can see the figure clearly. Figure 4a and 4b show light ray diagrams how and where the images are formed. In the diagram, light rays come from the left object O. When it enters the glass sphere, light bends according to the relative speeds in the two media. The red dotted lines are the normals to the refractive surfaces. As shown, when light slows down it bends towards the normal, and when it speeds up it bends away from the normal. The image is formed where the light rays meet. Comparing the two figures, shown in figure 4a the image I is formed before the retina, so the image is not in focus on the retina. While in figure 4b the image after the refraction is a virtual image I1, and is farther away to the left of the original object O. Your eye sees this virtual image as the object of the eye ball and can focus it at the retina.
So What?
A nearsighted (myopic) person cannot see distant objects clearly because the eye lenses refract the light too much and thus the image is focused in front of the retina, but he can see more clearly in the water because light refracts less from water to eye lenses and the image will be closer to the retina. In contrast, a person with normal eyesight is farsighted in the water, so he can see better wearing a pair of water-tight goggles with flat front surfaces.
If you match the light speeds in two different transparent media, that is to say their indices of refraction are made the same, then you won't be able to distinguish the two transparent materials. In our case, before you pour in the water you see the ball and some blurry background, but when you add water, which is a closer match to the glass, the glass ball nearly disappears and you see the object under the ball. This principle is used in packaging toothpaste, for example. How come the abrasive material in transparent gel toothpaste is invisible? The abrasive material is like the spherical glass ball: If you match the index of refraction of the gel and the abrasive material, you will not see the bits of abrasive material; they disappear in the gel.
Credit: I first saw this performed by Marshall Ellenstein of Maine West High School, Des Plaines, IL and it was published in The Physics Teacher, February, 1982.