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?

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In rigid body mechanics we presume that all objects are perfectly rigid. This means they change neither their form nor their volume when forces act on them. It simplifies the following mechanical analysis. Parts of human body are of course not perfectly rigid. Deformations would often make the analysis of motion too complicated. Fluid mechanics is concerned with mechanics of gases and liquids. Relativistic mechanics is related to the Einstein relativity theory and quantum mechanics describes behaviour of objects on atomic and subatomic level. In biomechanics we mostly make use of rigid body mechanics which is best applicable for describing the motion of human body and its parts. Since certain sport events take place in fluid environment, biomechanics also uses knowledge of fluid mechanics. Repeated little deformations of human body may cause injuries, therefore we will also partly inquire into mechanical deformation of human body and its parts.

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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.

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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 $450{\rm nm} \leq \lambda \leq 510{\rm nm}$.

Optical density may also mean the absolute value of the logarithm with base 10 of the power transmission factor of an optical attenuator:

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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.

Objects of interest in sport biomechanics are human body and sport equipment. According to the nature of studied objects mechanics is divided into several branches (Fig. 1).

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.

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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.

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.

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Figure 1 Branches of mechanics divided according to the nature of studied objects, and the division of rigid body mechanics.

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.

According to specific approach to the study of objects motion and their equilibrium, mechanics is divided into statics and dynamics. Statics studies objects that are either at rest, or in constant motion, that is a motion with constant velocity as to its magnitude and direction. Dynamics studies objects with acceleration. Dynamics is divided into kinematics and kinetics. Kinematics describes the motion of objects, while kinetics studies forces that cause changes of motion.

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.