What is the relationship between physical density and optical density?

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Discussion Overview

The discussion revolves around the relationship between physical density and optical density, exploring definitions and interpretations of optical density in the context of light transmission and material properties. Participants express confusion over the definitions and seek clarity on how these concepts interrelate.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants express confusion over two definitions of optical density: one related to the absorption and re-emission of light, and another concerning the ability of a medium to change the path of light.
  • One participant argues that optical density (OD) is not related to mass density, suggesting that OD measures a material's ability to pass light, with higher OD indicating less light transmission.
  • Another participant provides an operational definition of optical density, relating it to optical attenuation and giving an example of how it quantifies light reduction.
  • There is a discussion about the relationship between optical density and the speed of light in a medium, with some participants questioning whether an increase in OD correlates with a decrease in light speed.
  • One participant notes that conflicting definitions of optical density exist, with one equating it to opacity and the other to refractive index, which adds to the confusion.
  • Concerns are raised about the generality of the definitions provided, with some participants suggesting that they do not adequately capture the concept of optical density.
  • There is mention of specific cases, such as oil being more optically dense than water despite having a lower physical density, which raises questions about the relationship between the two types of density.
  • Some participants seek clarification on the mathematical definitions of optical density, noting variations in logarithmic bases used in calculations.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the definitions of optical density or its relationship with physical density. Multiple competing views and interpretations remain, particularly regarding how optical density is defined and its implications for light transmission and speed.

Contextual Notes

Participants highlight the ambiguity in definitions and the potential for confusion due to varying interpretations of optical density. The discussion reflects a lack of clarity on how physical density may influence optical density, with examples provided that challenge straightforward relationships.

Misr
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I read two definitions about optical density and I'm very confused about that

1.the sluggish tendency of the atoms of a material to maintain the absorbed energy of an electromagnetic wave in the form of vibrating electrons before re-emitting it as a new electromagnetic disturbance.

2.the ability of a medium to change the path of light


so what is the relation between these two definitions??

One more question:Is there a relation between the physical density and the optical density?Is it a direct relation ??
 
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First find better sources neither of those definitions are very good.

No, Optical Density is not related to mass density. Think of OD as a measure of a materials ability to pass light. The higher the OD the lower the amount of light which gets through. This will vary with wavelength for any given material.

A pair of laser safety glasses I have is rated; O.D. 7 @ 190-400nm; 1.3+ @488nm ; 5 @10.4nm.

They are the safety glasses you would wear for working around a UV laser.
 
First find better sources neither of those definitions are very good.
I find the first one good so what's wrong with these definitions?
Think of OD as a measure of a materials ability to pass light.
transparency?

The higher the OD the lower the amount of light which gets through
how?I can't Imagine this
 
Well yeah, basically it's transparency. Not exactly, and Integral's definition is better, but essentially.

So your third doubt should disappear.
 
What is wrong with the definitions you gave? If they were good you would not be here asking this question.

They are both way to general. Neither are direct definitions of OD, All transparent material effect the passage of light. The place where you will encounter OD in daily life is a pair of sunglasses. A very dark pair has a High OD thus letting through less light then a light pair. OD is a measure of how much light is adsorbed by the material. The more light (high OD) adsorbed by your glasses means less gets to your eyes.
 
Misr said:
I read two definitions about optical density and I'm very confused about that

1.the sluggish tendency of the atoms of a material to maintain the absorbed energy of an electromagnetic wave in the form of vibrating electrons before re-emitting it as a new electromagnetic disturbance.

2.the ability of a medium to change the path of lightso what is the relation between these two definitions??

One more question:Is there a relation between the physical density and the optical density?Is it a direct relation ??

Hi Misr. Googling "optical density" I see that unfortunately there are two (conflicting) definitions in common use. One (as per Integrals usage) which is synonymous with opacity (or absorbance) and another which is synonymous with refractive index. In other words, one definition refers to an optical material's ability to block light while the other refers to its ability to slow and bend light.

Both of your definitions however refer to the latter. The first is emphasizing the mechanism by which the light is slowed and the second is emphasizing that the ability to slow the light is also an indication of its ability to refract light (as per Snells Law)
 
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Let me provide an operational definition of optical density:
Write the number 1 and follow it with as many zeroes as the optical density. The number you get is the optical attenuation.
Example: after passing a filter of optical density 6, the outgoing light has been attenuated one million times.
Hope this is clear enough.
 
What is wrong with the definitions you gave? If they were good you would not be here asking this question.

They are both way to general. Neither are direct definitions of OD, All transparent material effect the passage of light. The place where you will encounter OD in daily life is a pair of sunglasses. A very dark pair has a High OD thus letting through less light then a light pair. OD is a measure of how much light is adsorbed by the material. The more light (high OD) adsorbed by your glasses means less gets to your eyes.
That's very simple but what about the relation between the OD and the speed?
as OD increases speed decreases ?
 
Misr said:
That's very simple but what about the relation between the OD and the speed?
as OD increases speed decreases ?
Did you read my reply #6?
 
  • #10
the second is emphasizing that the ability to slow the light is also an indication of its ability to refract light (as per Snells Law)
how?that's confusing me :(
 
  • #11
hello there??
 
  • #12
No, Optical Density is not related to mass density. Think of OD as a measure of a materials ability to pass light. The higher the OD the lower the amount of light which gets through. This will vary with wavelength for any given material.
see this page please
http://www.physicsclassroom.com/mmedia/waves/em.cfm
The actual speed of an electromagnetic wave through a material medium is dependent upon the optical density of that medium. Different materials cause a different amount of delay due to the absorption and reemission process. Furthermore, different materials have their atoms more closely packed and thus the amount of distance between atoms is less. These two factors are dependent upon the nature of the material through which the electromagnetic wave is traveling. As a result, the speed of an electromagnetic wave is dependent upon the material through which it is traveling.
doesn't this mean that there's a certain relation between the optical density and the physical density?
I know that oil is more optically dense than water although it has lower density
but I'm very confused :(
 
  • #13
can u please give me a proper definition of optical density . i will be thankful
 
  • #14
OD=lg(1/T) where T-transmission
But sometimes another definition is used OD=ln(1/T). So there may be confusion. To avoid it the base of logarithm should be specified.
 
  • #15
It's base 10.
 
  • #16
I met with use OD under e base also. May be it was inaccurate use.
 
  • #17
The base depends on the materials.

By liquids, the base is 10. By gases the base is natural. Check wikipedia, searching for transmittance...
 

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