Drunken Goggles

Most crystals are uniaxial or biaxial and thereby exhibit birefringence – that means that a ray of light which refracts in the object is divided into two rays. The two beams have different refractive indexes, whereby at least one of the refractive indexes is dependent on the orientation of the crystal. Moreover, both rays are orthogonally polarised to one another. These two properties can be used to construct a pair of goggles that can create a rotating double image with simple means.

The goggles consist of four components:
 the housing
 a polarisation filter
 a cylindrical bearing
 a birefringent substance, e.g. calcite; calcite is especially well-suited as it exhibits an extraordinarily strong birefringence and can also be synthetically produced
The cylindrical bearing allows the calcite to freely rotate within the housing. Only then is it possible to create a rotating image. One should also note that calcite is by nature a relatively soft stone and has a natural fission on the {1011} plane, which also gives calcite its typical form. Therefore it is advisable to leave the stone in its natural form and to embed it in another material. By inserting a linear polariser it is also possible to turn off the double image at times.