READING NESSE: Pages 121-125, Behavior of light in anisotropic minerals including interference colors / retardation and birefringence (difference in indices of refraction). The behavior of calcite is described in figure 7.13. If you don't remember what isotropic and anisotropic are, the definitions are on page 116. ASSIGNED: Also read these sections from Gregg Finn's Brock University Optical Mineralogy Page. < Double refraction of calcite--anisotropic minerals Packing of the crystal structure and it's effects on light We are going to discuss the double refraction of clear calcite (p. 122, fig. 7.13) called Iceland Spar and the strange properties of a mineral fondly called "TV Rock", whose real name is Ulexite. These two mineral examples do a great job of pointing out the difference between reflection and the critical angle versus refraction and Snell's Law. We'll also talk about those prism things that college kids hang in their windows to make pretty rainbows. How does that work anyway? ULEXITE--THE SECRET OF TV ROCK. While the behavior of calcite is explained in your book, ulexite isn't. Ulexite is a Borate with non-structural water (water not in the crystal structure). When ulexite has a fibrous habit, it forms in a lot of long, thin, cylindrical strands. The strands are packed closely together like a fiber optic bundle. If you cut the rock perpendicular to the strands, you would see a group of circular cross-sections packed very closely together. Still there is space between the strands that is filled with either water or air, which has a lower index of refraction. The circular section and the differences in index of refraction cause light that enters the fibers to be internally reflected. Thus, light enters from a variety of angles, but comes back up the strands bringing the image with it. Nature's own fiber optic cable! THINGS TO KNOW. Make sure that you know the difference between isotropic and anisotropic minerals. Know what causes light to split into a fast and a slow ray. Understand why this causes the waves to be out of phase upon leaving the crystal and produces interference colors (see fig. 7.14). Know what birefringence is and how to calculate it from indices of refraction and thickness. Why does calcite show double refraction and quartz doesn't?