-- You are looking at a thin box filled with small plastic discs. These discs are squeezed top-to-bottom and allowed to expand right-to-left. They are backlit, and the light passes through two linear polarizers, one behind the discs and one in front. The colors indicate the stresses inside the discs. -- To learn more about this experiment, see the American Physical Society's Physical Review Focus article Shear Forms Force Chains. See this video to watch the experiment in action.
