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Calcite crystals
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Try the calcite optical illusion!

You can use perfect calcite crystals to make an optical illusion called double refraction. Calcite has unique optical properties that split a ray of light into two beams that are bent at different angles because they travel at slightly different speeds. This splitting of light means that you see double when you look through a calcite crystal. It looks like there is two of everything!

Make a small drawing on a piece of paper, much smaller than your crystal of calcite. Place a transparent crystal of calcite on top of the drawing on the piece of paper. Spin the crystal slowly in a circle. In one position, your drawing will appear as a single image, but as you rotate the crystal, the drawing will gradually split into two images. Your drawing appears as a single image only when both light beams are traveling the same speed. This happens when light is traveling parallel to the primary axis of the crystal.

Last modified April 25, 2003 by Lisa Gardiner.

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The Fall 2010 issue of The Earth Scientist, focuses on rocks and minerals, including articles on minerals and mining, the use of minerals in society, and rare earth minerals, and includes 3 posters!

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