Optics. Crystal substances

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Some crystal substances did not satisfy to the refraction law, and split a ray of light on two, making two images instead of one. This phenomenon named double refraction, is noticed for the first time by Bartolinom (1625-1698) in 1669 a crystal of a carbonate of the calcium Used by Bartolinom, named the Icelandic spar, till now it is considered the best for demonstration of this phenomenon.

Could be, of course, so that for whatever reasons at a crystal it has appeared two indicators of refraction of light, and that one part refracts with one indicator, and another - with another. But measurements have shown that one of refractions at all does not submit to the law of Snella: at a hade equal to zero the refraction corner is not equal to zero.

About this image say that it is formed ' unusual ' by beams; the second image is formed by beams which satisfy to the law of Snella and are called ' as ordinary '.