Question

Optical density vs density

Answer 1

optical density is the ratio of speed of light in two media in contrast density is defined as mass per unit volume.

Answer 2

Optical density is a measurement of a refractive medium or optical component's ability to slow or delay the transmission of light.
On the other hand, Mass density is mass per unit volume. Optical density measures the speed of light through a substance, affected primarily by the wavelength of a given light wave. It is possible that mass density of an optically denser medium may be less than that of an optically rarer medium (optical density is the ratio of the speed of light in two media). For example, Mass density of turpentine is less than that of water but its optical density is higher.