A mirage


Using two identical parabolas we can create an interesting phenomenon. What you see is a sort of black box in the shape of a disc, with a flat hole on which a die is placed. You can light this die with a torch, and at first sight it has all the properties of a real object. But when you try to take or touch it, you see that there is no die: it's an optical illusion.

This mirage has a simple explanation. The box is actually made of two identical, superimposed parabolic mirrors, each of which has its focus more or less in the other's vertex. On the bottom of the lower mirror is the die, while the upper one has a hole through which it is possible to see inside. When one looks through this hole, or lights it up with a torch, the light ray makes two reflections before hitting the die, thus forming a real image of it, absolutely identical to the original.

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