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The Floating Pig
A famous illusion called the Mirage can be purchased from a variety of science stores. The illusion consists of two spherical mirrors facing each other. The top mirror has a hole in it. When you place an object on the bottom mirror, you see an image of that object (in this case a plastic pig) floating above the hole in the top mirror. The illusion looks very realistic and people are delighted and mystified when they reach out to grab the pig and come up with a handful of air!
Now imagine you take a standard laser pointer and shine it at the illusion of the pig. What will you see? Will the laser pass straight through the image of the pig or will the laser spot appear on the pig as if it were real? Why?
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Solution
First let's take a look at how the Mirage is formed. The Mirage consists of two parabolic mirrors with an opening in the top mirror. A small plastic pig is placed in the bottom of the mirror below the opening.
Two light ray are shown in the diagram. As you can see, the light rays converge at a point just above the opening forming the image you see.
When you shine the laser at the pig, the light will follow the exact same path at the rays in the diagram (except in the opposite direction) and hit the pig! Therefore, you will see the laser appear to reflect off the mirage of the pig!