by Dave Walker, UK
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Figure 6: Iceland spar cleavage rhomb. Effect
of rotating the rhomb 360° above an image (eight images were captured at 45° intervals).
There is a single 'A B' printed on the paper below the rhomb. The double image is caused by the marked birefringence or 'double refraction' of this form of calcite. As the crystal rotates, the 'A B' from the 'extraordinary' image path rotates around that from the 'ordinary' image path. Although not clearly seen here, the two images of 'A B' appear to be at different heights in the calcite; this is because the two light paths have different velocities i.e. different refractive indexes.
(8 jpeg images, each 14kbytes, at 45° rotation intervals.)
Note: This illustrates how a sequence of jpeg images can be used to demonstrate the effect rather than a detailed explanation of the effect itself. Readers unfamiliar with this classic demonstration can consult an introductory text on polarized light microscopy or online resource such as Molecular Expressions: Optical microscopy primer.
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