Question

What is optical density and physical density ?

Answer 1

Hint:Optical density is related to the optics ( Optics is the science which deals with the light and phenomenon that happens due to light such as interference of light, scattering of light etc.)
Physical density is defined for the concentration of mass of an object per unit volume.

Complete step by step answer:
Optical density- It is the measure of the transmittance through an optical medium. Optical density is the process of light or other electromagnetic radiation by matter.

The process of emission and absorption depends on the wavelength of the radiations, which includes the interaction between fundamental particles like electrons, atoms, ions etc.

When a beam of light interacts with absorbing atoms, absorption takes place.
Absorption depends on the thickness and the concentration of the absorbing atoms.

Mathematically it is the logarithmic ratio of the falling radiation to the transmitted radiation through a material.
The measurement is done at maxima of the absorbance spectra as there is the least chance of absorbance with the change in wavelength. The measurement is a common method to quantify various important parameters like concentration of cell, production of biomass etc.

Physical density- density is a measure of mass per unit of volume. The average density of an object equals its total mass divided by its total volume.
The physical density is also known as volumetric mass density.

An object which is made up of comparatively dense material such as metal, alloy of metals etc will have less volume than an object made of less dense material such as plastic, cotton etc for equal amount of mass.

Note:-
The higher is the optical density, the lower is the transmittance.
The physical density of the material varies with the temperature and pressure. This variation is very small for the solids and liquids but in case of gases the physical density varies by much.