Suspended particles and the index of refraction

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Discussion Overview

The discussion centers on whether suspended sub-micron sized particles influence the index of refraction of a liquid, particularly in the context of visible light and scattering phenomena. It explores the complexities of Mie Scattering and the implications of particle size on refractive measurements.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that the influence of suspended particles on the index of refraction is a complex scattering problem, particularly relevant to Mie Scattering due to the size of the particles relative to the wavelength of light.
  • One participant notes that filtering out particles results in a lower reading on a refractometer, attributing this change to the elimination of scattering, while questioning the predictability of this effect quantitatively.
  • Another participant inquires about the size of the particles in relation to the wavelength of light, seeking clarification on whether they are smaller than the wavelength.
  • A participant describes their use of a handheld optical refractometer and observes that the reading is higher when the liquid is cloudy, prompting questions about the underlying reasons for this observation.
  • One participant proposes a theoretical explanation involving the behavior of particles smaller than lambda/pi, suggesting that such particles induce a lagging current due to the incident electric field, which could lead to an increased index of refraction.
  • A later reply seeks clarification on the term lambda/pi, questioning if it refers to roughly one-third the wavelength of light, while noting that the particles in question are larger than 450nm.

Areas of Agreement / Disagreement

Participants express varying viewpoints on the influence of particle size and scattering on the index of refraction, with no consensus reached on the exact mechanisms or implications. The discussion remains unresolved regarding the specific effects and theoretical explanations proposed.

Contextual Notes

There are limitations regarding the assumptions made about particle size and its relationship to the wavelength of light, as well as the dependence on the definitions of terms used in the discussion. The complexity of the scattering phenomena and the specific conditions of the refractometer measurements are also noted but not fully resolved.

PenderJ
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Do suspended sub-micron sized particles influence the index of refraction of a liquid?
 
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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:
 
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 .
 
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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?
 
When you say sub micron, are we talking about smaller than the light wavelength?
 
What type of refractometer? The answer to your question is very probably yes. Is the liquid with particles still nearly transparent?
 
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?
 
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.
 
tech99 said:
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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