Question

differentiate between mass density and optical density

Answer 1

Optical density should not be confused with mass density, which ismass per unit volume. It is possible that mass density of an opticallydenser 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, turpentine and water.

Answer 2

Mass density is mass per unit volume
optical density is the ratio of the speed of light in two media
Example turpentine here mass density is less n optical density is high