Question: I'm a zoology minor and we are doing protein estimation by colorimetric method. I have stumbled upon a term 'Optical density'. I don't understand the term well. Is it a measure of the extent of light that can pass through a particular object?

I've checked a [related question](https://physics.stackexchange.com/questions/132094/why-does-the-optical-media-have-different-refractive-indices) of this community and it doesn't solve my question completely.

Answer: 
For all intents and purposes, OD is the negative of the order of magnitude the factor by which the intensity of the light is reduced by the attenuating element with said OD.

In other words: OD = 6, means that the intensity will be reduced by a factor of 10 to the power of -6, a.k.a by a factor of a million.
The term Optical Density or ("OD") can have two different but related uses.

1. The first is in relation to the refractive index. I've included an image that will enable a quick grasp of this concept.

[![REFRACTIVE INDEX EXPLAINED](https://i.stack.imgur.com/JUUSe.jpg)](https://i.stack.imgur.com/JUUSe.jpg)

2. The second use is in reference to to the following:

> 
> Optical density may also mean the absolute value of the logarithm with
> base 10 of the power transmission factor of an optical attenuator:
> 
> 
> For example, an optical density of 3 means that the optical power is
> attenuated by the factor $10^3$ That would correspond to an
> attenuation by 30 decibels.
> 
> 
> If several attenuators are used in series, their optical densities can
> simply be added.
> 
> 
> To avoid ambiguity, it is better to use the term absorbance.
> 
> 
> 

<https://www.rp-photonics.com/optical_density.html>

Author: Dr. Rüdiger Paschotta - <https://www.rp-photonics.com/paschotta.html>

To further the explanation above, I thought this image describing the second usage of optical density may be useful. The concept is being applied to a lens in regards to a laser passing through it. Each wavelength in the following chart will have its own OD but the notation is stating the minimum optical density over a range of wavelengths.

[![Laser Safety Optical Density Explained](https://i.stack.imgur.com/RyQfU.jpg)](https://i.stack.imgur.com/RyQfU.jpg)