Optical properties of sea ice depend to a greater or lesser extent on its crystalline properties and on the size, shape, and distribution of brine inclusions systematically trapped in the ice crystals. Here we demonstrate the use of polarized light techniques to examine the internal structure of sea ice. Using both naturally occurring and laboratory simulated sea ice we show how the crystalline and salinity components originate including discussion of the mechanisms by which first-year ice desalinates and recrystallizes into multi-year ice exhibiting optical properties significantly different from those of first-year ice.