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What is optical density?

  1. Sep 4, 2011 #1
    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 ??
  2. jcsd
  3. Sep 4, 2011 #2


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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.
  4. Sep 4, 2011 #3
    I find the first one good so what's wrong with these definitions?

    how?I can't Imagine this
  5. Sep 4, 2011 #4
    Well yeah, basically it's transparency. Not exactly, and Integral's definition is better, but essentially.

    So your third doubt should disappear.
  6. Sep 4, 2011 #5


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    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.
  7. Sep 4, 2011 #6


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    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)
    Last edited: Sep 4, 2011
  8. Sep 4, 2011 #7
    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.
  9. Sep 5, 2011 #8
    That's very simple but what about the relation between the OD and the speed?
    as OD increases speed decreases ?
  10. Sep 5, 2011 #9


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    Did you read my reply #6?
  11. Sep 5, 2011 #10
    how?that's confusing me :(
  12. Sep 7, 2011 #11
    hello there??
  13. Sep 7, 2011 #12
    see this page please
    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 :(
  14. Jan 27, 2012 #13
    can u plz give me a proper definition of optical density . i will be thankful
  15. Jan 27, 2012 #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.
  16. Jan 27, 2012 #15
    It's base 10.
  17. Jan 28, 2012 #16
    I met with use OD under e base also. May be it was inaccurate use.
  18. May 4, 2012 #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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