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Question:
Does the temperature of a liquid have an effect on the refractive index?



Replies:
The "short" answer is yes, because temperature affects the density of the liquid, which in turn, affects its index of refraction. Quantitatively, just how much and in what direction is more difficult to say. If one has to "guess" it would seem that as the temperature increases, the density decreases so that the speed of light "should" approach that of the speed of light in a vacuum. But that is only an estimate, and the change would probably be quite small.

Vince Calder


Yes, the refractive index (RI) of a liquid often is different at different temperatures (usually negative; e.g. RI goes down as temperature goes up), although typically the effect is very small. For example, water changes less than 0.01% per degree Celsius.

However, that is not to say that there is a constant relationship between RI and temperature. Refractive index is proportional to the square roots of electrical permittivity and magnetic permeability. These factors may change with temperature, but not linearly, and therefore RI does not have a simple relationship with temperature.

For correlation data, there is a lot of published literature on the web discussing various materials -- search for the term 'temperature coefficient' and 'refractive index' together, and you will have information coming out your ears. For more of the basic physics, I would first study permittivity and permeability, and then apply these concepts to your liquids of interest.

Hope this helps,

Burr Zimmerman


I think the average clear liquid has small index changes with temperature, mostly due to volume expansion with temperature, and of roughly similar percentage.

The wave-slowing contribution of the liquid over and above that of free space, that is, the part of the index above 1.0, should vary with the same rate as the density does. (Temperature coefficients are often stated in ppm/degC: "ppm" = parts-per-million ; 1 ppm = 10^(-6) = 0.0001 % .)

That all presumes that the liquid is single substance that does not change its chemical composition. Such as water, or a single stable solvent. Some equilibrium mix might have additional larger changes if the equilibrium shifts substantially with temperature.

A given liquid expands more rapidly near its boiling point than well under boiling, and liquids near their critical temperature expand a lot faster.

Go ahead and look up the temperature coefficient of expansion of your liquid, (or a table of its density versus temperature, easier to find,) and presume the index change is similar.

For water, the tempco (of volume expansion) is about 200ppm/C at room temperature, rising to about 700ppm/C approaching100C.

For ethanol, it is about 1000ppm/C at room temperature. So I think ethanol's index, being around 1.5, might change about 0.3% for 10 degrees C. Just getting large enough to notice.

Jim Swenson



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