Refractive Index - A Mixing Question Answered

[lionelwang]

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

 

[sophiecentaur]

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.

 

[lionelwang]

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.

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.

 

[sophiecentaur]

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".

 

[Claude Bile]

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.