by Dave Walker, UK
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Figure 7: Iceland spar cleavage rhomb. Effect on the double image of rotating a polarizing filter 360° above the crystal.
There is a single 'A B' printed on the paper below the rhomb. In normal light a double image is seen which is caused by the marked birefringence or 'double refraction' of this form of calcite (see Fig. 6). In the sequence above there is a rotating polarizing filter above the rhomb. The filter was initially set to give one single strong 'A B' image, then the filter was rotated 45° between each image still in the animation.
The animation demonstrates that the 'ordinary' and 'extraordinary' ray paths are polarized
and at right angles. The polarizing filter extinguishes each 'A B' image
when its plane of polarisation is at right angles to that of the filter. Note
the weaker double image when the polarizing filter is not at right angles
to either of the 'A B' images from each ray path.
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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