Suspended particles and the index of refraction

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  • #1
PenderJ
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Do suspended sub-micron sized particles influence the index of refraction of a liquid?
 

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  • #2
PenderJ
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I guess it isn't an intermediate question. How do you edit it to change the level? Is there a level that corresponds to it being too hard for this forum? :smile:
 
  • #3
hutchphd
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For visible light this is a scattering problem of some complexity: ##1\mu m=10^4Angstrom##so the particles are slightly larger than the wavelength so this is the realm of Mie Scattering and not just the index of refraction which is usually applied to more homogeneous materials. Certainly the physics is interesting .
 
  • #4
PenderJ
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So when I filter out these particles and observe a lower reading in a refractometer I'm actually observing a change due to the elimination of scattering. And predicting this quantitatively would be complicated. Is that correct?
 
  • #5
tech99
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When you say sub micron, are we talking about smaller than the light wavelength?
 
  • #6
hutchphd
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What type of refractometer? The answer to your question is very probably yes. Is the liquid with particles still nearly transparent?
 
  • #7
PenderJ
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I'm using 0.45 micron filters. The liquid is translucent and a little cloudy prior to filtration and has good clarity afterwards. The liquid is espresso coffee.

I have an el cheapo ($25) hand held optical refractometer, the kind where you look for the shadow line through the eyepiece. Prior to filtration the line is fuzzy but it is also higher on the scale. I have read about the same phenomenon (higher reading, less precise) being observed with more expensive electronic refractometers.

I was just curious what exactly was going on to change the reading. I get the fuzzy part, being that the liquid is cloudy. But why would it read higher?
 
  • #8
tech99
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This is what I suggest is happening. If the particles are smaller in diameter than lambda/pi then they are too small for resonance. This means they are inductive. The incident E-field causes a circumferential current on the surface of the particle and this current lags the E-field by 90 degrees. The current results in radiation which is 90 degrees delayed relative to the incident E and B waves. So the radiation from the particles is slower than free space. This gives the material an increased Index of Refraction.
 
  • #9
PenderJ
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This is what I suggest is happening. If the particles are smaller in diameter than lambda/pi then they are too small for resonance. This means they are inductive. The incident E-field causes a circumferential current on the surface of the particle and this current lags the E-field by 90 degrees. The current results in radiation which is 90 degrees delayed relative to the incident E and B waves. So the radiation from the particles is slower than free space. This gives the material an increased Index of Refraction.

Excuse my ignorance but by lambda/pi do you mean roughly 1/3 the wavelength of light? The particles I'm talking about are larger than 450nm.
 

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