What is the Difference Between Mass Density and Optical Density

The key difference between mass density and optical density is that mass density is the mas per unit volume of a substance, whereas optical density is the speed of light in a medium.

Density is a common term that we usually use to describe the relationship between the mass and volume of a particular substance. Mass density and optical density are two derivatives.

CONTENTS

1. Overview and Key Difference
2. What is Mass Density 
3. What is Optical Density
4. Mass Density vs Optical Density in Tabular Form
5. Summary – Mass Density vs Optical Density 

What is Mass Density?

Mass density can be described as the mass per unit volume of a substance. It is a significant characteristic of matter. The mass density is usually directly connected to mass. Moreover, mass is a measurement of the inertia of an object.

For a bulk material with uniform mass distribution, we can easily calculate this parameter by dividing the total mass of the object by the total volume occupied. However, if the mass distribution is not even, we need more complicated methods to measure the density.

Furthermore, we can easily describe the floatation of a substance using its density. Here, floatation means that a fluid or a uniform solid that is denser than a given fluid will drown in the given fluid. Thus, if the density of the fluid or the uniform solid is lesser than that of the given fluid, it will float on the given fluid. Moreover, we can define the term relative density in order to compare the densities of the two fluids. This is the ratio of the two densities and is merely a number.

What is Optical Density?

Optical density or OD is the degree to which a refractive medium retards transmitted rays of light. This means that optical density is a term that describes the propagation of a light wave through a substance. The measurement of optical density is taken as the logarithmic ratio between incident radiation on the substance and the radiation transmitted by the substance. Therefore, optical density influences the speed of light through a substance. The main factor affecting optical density is the wavelength of the light wave.

It is important to note that optical density has no relationship with the physical density of the substance. Optical density expresses the tendency of the atoms or molecules of a substance to retain the absorbed energy. This retention occurs via electronic vibrations. Therefore, if the optical density of a substance is high, the speed of light through this substance is low (since light waves move slowly). Furthermore, optical density can be measured using a spectrometer.

The refractive index of a material indicates the optical density of that substance. To be more specific, the ratio between the speed of light in a vacuum and the speed of light through the substance gives the refractive index. This explains how slow the speed of light in a substance is in comparison to that in a vacuum.

What is the Difference Between Mass Density and Optical Density?

Mass density and optical density are related to each other. Generally, the mass of a substance is inversely proportional to its optical density. The key difference between mass density and optical density is that mass density is the mass per unit volume of a substance, whereas optical density is the speed of light in a medium. The unit of measurement of mass density is g/cm³ or kg/L while the unit of measurement for optical density is AU/cm.

Below is a summary of the difference between mass density and optical density in tabular form for side-by-side comparison.

Summary – Mass Density vs Optical Density

The key difference between mass density and optical density is that mass density is the mass per unit volume of a substance, whereas optical density is the speed of light in a medium. Generally, the mass of a substance is inversely proportional to its optical density.

Reference:

1. “Optical DensDensity spectra of human lensity.” Chem Europe.

Image Courtesy:

1. “Density column” By PRHaney – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Density spectra of human lens”  By Zyxwv99 – Diagram made in Inkscape 0.48 (CC BY-SA 3.0) via Commons Wikimedia