The 3-D mirascope consists of two identical parabolic mirrors that face each other. The top mirror has a hole in the center where the reflected image of an object appears. To create the image you place an object in the bottom of the bottom mirror. This bottom mirror, combined with the identical mirror placed on top, project a real-looking image in the opening of the top mirror. It's an illusion but it looks completely real! You can use any object you like, such as a coin, a dice, or a rock, and it will completely fool your friends and family.
Check out a video of a mirascope:
For this trick to work, the two mirrors have to be identical, with the only difference being that the top mirror has a hole in the center.
The object placed at the bottom of the bottom mirror is located at the focal point of the top mirror. As a result, the top mirror reflects the light from the object such that the reflected light rays are parallel and pointing directly towards the bottom mirror. These light rays then reflect off the bottom mirror and focus towards the focal point of the bottom mirror, which coincides with the location of the top hole. At this location the image of the object is formed. The mirrors essentially "copy" the image of the object, and because the mirrors are identical the image is exactly the same size and shape as the object, so we think we are looking at the object but instead we are looking at the reflected image of it.
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