Complex sentence: When only an isotropic material such as air, water, or glass exists between the filters, all light is blocked, but most crystalline materials and minerals change the polarizing light directions, allowing some of the altered light to pass through the analyzer to the viewer.
Split into simpler sentences: When only an isotropic material such as air, water, or glass exists between the filters, all light is blocked. However, most crystalline materials and minerals change the polarizing light directions, which allows some of the altered light to pass through the analyzer to the viewer.