what is the difference between optical density and mass density?
Optical density is a term applied to how well light passes through a material. If light is little affected passing through it, that is, if almost all the light goes through it that goes into it, it has a very low optical density. Like clear glass. Very low optical density. Start coloring the glass and less light can get through it; it has a higher optical density than clear glass. The amount of light that can get through a material is addressed in its transmittance, and the amount of light that is "blocked" by a material is addressed in its opacity.
optical density = speed of light / speed in medium
mass density = mass / volume