Clarify the meanings of optical density and mass density.

(N/A) Mass density is defined as mass per unit volume $( \rho = m/V )$.
Optical density is a measure of the ability of a medium to refract light, often related to the refractive index of the medium.
It is important to note that mass density and optical density are not the same.
It is possible for an optically denser medium to have a lower mass density than an optically rarer medium.
For example, the mass density of turpentine is less than that of water, but its optical density is higher.