We are grateful to the UW Geology Department for providing the thin section of rock shown above.  You are looking at a slice of rock about 30 microns thick, illuminated from behind by polarized light.  So, the first thing to recognize is that if you make a slice of rock that is thin enough, you can see through it.  Except around the edges, where it is opaque, this particular rock is equally transparent to all colors--if it absorbed one color particularly well, we would see it as being the complementary color.  Yet when the rock is viewed through a plane polarizer, all kinds of colors appear, and they change as the polarizer is rotated (play video to see).  Remember the illumination is already plane polarized.  Furthermore, if the plane polarizer is fixed, and the rock is rotated, the colors change.  On top of that, if the plane polarizer and the rock are fixed, and you move your eyes around, the colors change.  So:  this week’s physics puzzler is--what happens to the light inside the rock?