# Does a fluid's temperature affect the way light passes through it?

For example, if I were to supercool water would it's refractive index still be 1.33 or would it be 1.31, the same as water-based ice even though it's still in liquid form?

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Yes, refractive index is usually temperature dependent. I'm guessing here that the index of supercooled water would be different than room temperature water, but also different from ice, because the density is different in each case.

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Yes, index of refraction in general depends on temperature. (Note that it also depends on wavelength.) My copy of the CRC Handbook gives the following for liquid water using a He-Ne laser with $\lambda = 632.80$ nm:
$n = 1.31744$ at 100 C, $n = 1.33306$ at 0 C, with steps of 10 C in between.