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: 
You're a little confused probably because there are *two* usages of the words "optical density". 

The first usage is as a synonym for *refractive index*, as described in the answers to the related question you cite. This is the commoner usage in physics.

The second usage is the total attenuation afforded by a protective screen, neutral density filter, laser goggles or the like. $ODx\; \lambda=y$ or even $ODx\; y$ means that the filter, goggles etc afford a power attenuation factor of $10^x$ at a light wavelength of $y$ or light wavelength range $y$. That is, the power transmitted through the filter is $10^{-x}$ of the incident power when the wavelength is as stated.

For example, laser goggles marked $OD7\;488{\rm nm}$ means that the goggles will reduce incident power at 488nm by a factor of $10^7$. Goggles marked with a lone wavelength rather than a wavelength range are always meant for use with a particular kind of laser. For example, the $OD7\;488{\rm nm}$ goggles are meant for use with an argon ion laser. You cannot rely on them using another source of wavelength 485nm, for example. 

For generic use, a wavelength range must be specified. So, for example, one often sees $OD7\;450{\rm nm} - 510{\rm nm}$, meaning, pretty obviously, goggles that will give you seven orders of magnitude of attenuation over the whole range $450{\rm nm} \leq \lambda \leq 510{\rm nm}$.
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)