Let’s try optics again What we have learned to date Refractive Index Snell’s Law And how it works Birefringence or Interference Colors Absolute value of the difference in refractive indices Isotropic vs Anisotropic minerals What we covered in lab Color and pleochroism (plane light) Anisotropism and interference colors (crossed polars) Extinction angles How to recognize twinning Relief Interference colors - review From Relief to the Becke Line ngrain >noil noil = 1.525 ngrain = 1.60 Figures from http://www.gwu.edu/~forchem/BeckeLine/BeckeLinePage.htm N of the oil greater than that of the mineral grain Figures from http://www.gwu.edu/~forchem/BeckeLine/BeckeLinePage.htm Becke line again The Becke line results from the concentration of light either inside or outside of the image of the particle, depending on whether the mineral grain or the oil has the larger index of refraction. This refraction of light at the boundaries creates an optical halo perceived as the Becke line. This halo is caused by the concentration of refracted light rays along the edge of the particle . As you lower the stage or raise the tube, the Becke line will move toward the region with higher index of refraction. Let’s revisit calcite What do we know about calcite to date a = 4.989, c = 17.062 Light splits into two rays traveling at two different velocities We can look up that n1=1.486, n2=1.64-1.66 Thus the birefringence=0.1540-0.1740 Shows very high interference colors Slide from Jane Selverstone fast ray Some light is now able to pass through the slow ray upper polarizer mineral grain When light gets split: -velocity changes -rays get bent (refracted) -2 new vibration directions plane polarized -usually see new colors light W E lower polarizer What we don’t know about calcite In which directions the rays travel in the mineral. How are these directions related to the crystal lattice? Some new stuff!! ε –epsilon known as the extraordinary ray ω omega known as the ordinary ray Where they travel Ordinary ray ω vibrates perpendicular to the c-axis Extraordinary ray ε vibrates perpendicular to the ordinary ray in a plane that contains the c-axis One of these rays travels faster than the other What happens in calcite again? calcite calcite ordinary ray, w extraordinary (stays stationary) ray, e (rotates) Slide from Jane Selverstone What happens if I orient calcite, so light passes only along the C-axis? I will see only one dot! The c-axis in calcite coincides with what we call the OPTIC axis. Birefringence is zero for light traveling along the c-axis In actual fact all hexagonal and tetragonal minerals behave the same way in polarized light. There will be one optic axis—thus known as Uniaxial Some definitions If n of ε is greater than n of ω, then the mineral is positive If n of ε is less than n of ω, then the mineral is negative Thus in a positive mineral, omega is faster than epsilon (velocity and n are inversely related) How could we determine if calcite were positive or negative? What information do we need?