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Double Refraction

المؤلف:  GEORGE A. HOADLEY

المصدر:  ESSENTIALS OF PHYSICS

الجزء والصفحة:  p-489

2025-12-23

710

If a crystal of Iceland spar AB (Fig. 1) is placed upon a sheet of paper on which there is a black dot, the eye placed above. the spar will see two dots, one a short distance from the other and apparently above the surface of the paper. This separation of the light into two rays shows that light passes through the crystal more rapidly in one direction than in the other and is called double refraction.

Two rays from each point reach the eye by different paths: one of them obeys the ordinary laws of refraction, and is called the ordinary ray; the other does not obey these laws, and is called the extraordinary ray. Another peculiarity of these rays is that they are polarized.

If a crystal of Iceland spar is cut along the diagonal A В and then cemented together again with Canada balsam, it has the property of permitting only the extraordinary ray to emerge. This arrangement is called a Nicol's prism, and is much used in the study of polarized light.