Refractive Index - A Mixing Question Answered

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Discussion Overview

The discussion revolves around the refractive index of mixtures formed from two components with known refractive indices, n1 and n2. Participants explore whether the refractive index of the mixture can be expressed as a weighted average of the two indices, considering potential chemical reactions and other factors that may affect the optical properties of the mixture.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant proposes that the refractive index of a mixture could be calculated as n = a*n1 + b*n2, where a + b = 1, assuming the components are fully mixable.
  • Another participant suggests that if there is a chemical reaction or significant bonding between the molecules, the proposed equation may not hold true.
  • A later reply mentions that factors such as solid state changes, nonuniform density, or phase separation could invalidate the equation for calculating the mixture's refractive index.
  • One participant notes that graded index optical fibers likely utilize variable mixtures, implying that the relationship may sometimes be valid.
  • Another participant asserts that for a pure mixture without reactions, the equation can be applicable to some extent and references a source for calibrated refractive index oils.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of the weighted average equation for calculating the refractive index of mixtures. Some agree that it may work under certain conditions, while others emphasize the limitations posed by chemical reactions and other factors, indicating that the discussion remains unresolved.

Contextual Notes

Limitations include the dependence on the assumption of complete mixability, the impact of chemical reactions, and the potential for phase separation affecting the optical properties.

lionelwang
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Hi all,

I am thinking about the refractive index, if I have two components with refractive index n1 and n2, respectively, then I mix them up (chemical reactions might happen after the mixing), and should I expect a mixture with refractive index:n=a*n1 + b*n2, where a+b=1.
In other words, should the mixture refractive index (n) be somewhere between n1 and n2?

Need help, thank you very much!

Regards
 
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If the two substances are totally 'mixable' (? right word ?) then that would work, I think. For gases, I guess you could assume it. If there is a chemical reaction during mixing, or even significant bonding between the two sets of molecules then you couldn't rely on it I'm sure. Also, you could get an emulsion which could affect the optical properties by making the mixture cloudy.

The refractive index for microwaves works that way, though. Varying the density of foam by altering the bubble size ( your a and b, effectively) can alter the refractive index and is used in coax cable and some microwave lenses, I believe.
 
After I did some google work, I found that, approximately, that equation works, but yes, as you said, the change of the solid states, nonuniform density, or phase separation, etc. which will make the equation not valid for caculation of the mixture refrective index.

Thank you very much for your great answer.


sophiecentaur said:
If the two substances are totally 'mixable' (? right word ?) then that would work, I think. For gases, I guess you could assume it. If there is a chemical reaction during mixing, or even significant bonding between the two sets of molecules then you couldn't rely on it I'm sure. Also, you could get an emulsion which could affect the optical properties by making the mixture cloudy.

The refractive index for microwaves works that way, though. Varying the density of foam by altering the bubble size ( your a and b, effectively) can alter the refractive index and is used in coax cable and some microwave lenses, I believe.
 
When they make graded index optical fibres, they must be using some variable mixture to get the grading so I guess the answer must be "sometimes you can".
 
lionelwang said:
Hi all,

I am thinking about the refractive index, if I have two components with refractive index n1 and n2, respectively, then I mix them up (chemical reactions might happen after the mixing), and should I expect a mixture with refractive index:n=a*n1 + b*n2, where a+b=1.
In other words, should the mixture refractive index (n) be somewhere between n1 and n2?

Need help, thank you very much!

Regards

Given this statement, then no, you would not expect the refractive index to relate to the reactants at all.

For a pure mixture though (no reactions), this does hold true to a degree. Check out Cargille's website; they make a whole bunch of calibrated refractive index oils.

Claude.
 

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