Why Does Wet Toilet Paper Look See-Through? The Science Explained! Dry paper looks opaque because its cellulose fibers are interspersed with tiny air gaps, which cause light to scatter due to the large difference in refractive index between air (~1.0) and cellulose (~1.5). When water fills these gaps, its refractive index (~1.33) is much closer to that of cellulose, so light bends less at the boundaries and scattering is greatly reduced. As a result, more light passes straight through the paper,