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How does optical density relate to mass density and refractive index?

  1. Aug 24, 2013 #1

    I am preparing a lesson on refraction and I've come across a confusing point. In a book I have read, it was said that the optical density of a material is proportional to the refractive index. That is, the greater the optical density, the greater the refractive index. I have however come across a different definition of optical density - ie. it is a measure of how much light the material will transmit - and this definition obviously has no relation to refractive index. Which definition is correct here?

    Finally, from my own experiences, I believe that there is commonly a relation between mass density and refractive index. That is, the denser the material, the greater the refractive index. For example glass is denser than water is denser than air is denser than vacuum and their respective refractive indices have a similar relation. Can someone please comment on this anecdotal rule? - and please give examples of any exceptions to the rule.

  2. jcsd
  3. Aug 24, 2013 #2
    Taking the second point first...

    This "rule" is often true, and presumably led to the words "optical density" being used in the context of refraction. However there are many exceptions as suggested by this Wikipedia entry. Perhaps the easiest to observe is vegetable oil and water.

    This is correct. "Optical density" means two totally different things. To avoid confusion you can always use "refractive index" in one context and "absorbance" in the other, but note that in the UK "optical density" is used in some GCSE Physics materials in the context of refraction so you may need to explain this use.
  4. Aug 24, 2013 #3


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    There are two different properties, both are called "optical density" in different fields of optics. That's unfortunate, but there is no "right" and "wrong" definition.

    That is a general trend, but denser materials (as in mass/volume) don't have to have a larger refractive index. As an example, ethanol has a slightly higher index than water, but it is less dense.

    Edit: I was too slow.
  5. Jan 2, 2016 #4
    Mass density is the number of molecules present in unit volume.They represent how heavy or light an object is based on their mass density.An object with more number of molecules within a unit space is more denser and vice versa.oil with larger molecules present in it find it difficult to accommodate more number of molecules within a unit volume while the water can since it has small molecules.Thus we see oil float it water because it has less mass density than water.
    Coming to optical density,it is related to the speed of light in a particular medium.The more optically denser the medium,lesser the speed of light and vice versa.We often confuse ourselves thinking that light travels slower in a medium with optically denser medium because it has more molecules within a unit volume that does not allow light to travel faster.That is wrong. Kerosene is lesser in mass density than water but is optically denser than water.
    I still have to figure out why speed of light is faster in a medium with less optical density.
  6. Feb 19, 2016 #5
    Water forms covalent bonds between molecules. Water molecules are closely packed (greater mass density than Kerosene). But Kerosene has more atoms than water per molecule but the molecules more spread out. The result is that stress energy tension (curvature of space) in water is better evenly spread, while stress energy tension is a wide range in Kerosene. This causes photon trajectories to "wobble" more. Sort of a zig-zag path.
  7. Feb 19, 2016 #6


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    That is an oversimplification. The atomic masses of the atoms is the main factor in the density of a substance.
  8. Feb 19, 2016 #7
    No, water does not form covalent bonds between molecules.

    The rest is not even wrong. :)
  9. Feb 19, 2016 #8


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    That does not make sense at all.

    This thread is from 2013, time to end it.
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