Give one example of a substance whose mass density is low but optical density is high.

(N/A) Optical density is a measure of how much a medium slows down the speed of light,whereas mass density is the ratio of mass to volume. Turpentine oil is a classic example of a substance that has a lower mass density than water but a higher optical density (refractive index) than water. Specifically,the mass density of turpentine is approximately $870 \ kg/m^3$ (less than water's $1000 \ kg/m^3$),while its refractive index is approximately $1.47$ (greater than water's $1.33$).