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.
Optical density is a mathematical way of describing the extent of attenuation of visual lightwaves.

Density is the log of opacity.

Opacity is the reciprocal of transmittance.

So a transmittance of 50% of the light (half of the light) would have an opacity of 2. The log of 2 = .3

So a density of .3 blocks half of the light energy.