Question

What is the difference between Optical Density and Mass Density? Give examples.

Answer 1

Optical density (OD) of a medium is the measure of its transmittance for a radiation of particular wavelength. Higher is the value of OD, lower is transmittance. It is expressed as log10 (1/T), where T is the transmittance in percentage.

OD is related to refractive index of a material. A higher value of refractive index means higher OD. This means that light travels slower, in a medium with high optical density.

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The mass density of a material is defined as ratio of the mass of the material to volume it occupies. So, in effect, mass density tells us how tightly or loosely a material is packed.