Natural crystals have long held a close relationship with several types of perspective; both for forming and magnifying images of spatial reality (visual and instrument perspective), and, in modern times, for creating unusual optical effects that lead to such instruments as LCD displays and CCD detectors.
Over 2000 years ago, natural crystals such as quartz or even natural glass lumps were polished into lens shapes to magnify objects and were also used to project images, which would have been a natural experiment anyone would have done with such an object. Evidently, we can see how the special and unusual optical properties of crystals have underpinned the development of various kinds of artificial perspective; and for viewing, matching, and representing spatial reality.
Crystal Optics
Crystal optics, or optical crystallography, is the science of how light behaves in anisotropic media like crystals.
Unlike transparent glass, where light behaves the same in all directions, a crystal’s highly ordered atomic structure means the speed, refraction, and polarisation of light change depending on the viewing angle and direction.
Birefringence (Double Refraction)
The most striking optical property of many crystals is birefringence (double refraction). When unpolarized light enters a crystal (such as calcite), the atomic lattice splits the light into two distinct rays, each polarised at a 90-degree angle to the other.
- The effect: If you place a clear calcite crystal over a piece of text, you will see a double image through the crystal.
- The optic axis: Every birefringent crystal has one or more special directions called the optic axis. Light travelling strictly parallel to this axis does not split, whereas light travelling at other angles experiences maximum separation.
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