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In my school book its stated that:

With increase in temperature, the speed of light in a medium increases, so the refractive index of medium decreases.

I read in another answer that:

Refractive index does not change because of a change in temperature, it changes because of a change in density. Increased temperature usually reduces density...

But if I quote another line of my Physics book, it contradicts the above answer by stating that:

While passing from one medium to the other, if light slows down, the second medium is said to be optically denser than the first medium and if light speeds up, the second medium is said to be optically rarer than the first medium. However, optical density has no relation with the density of medium. Kerosene is less dense than water but its optically denser than water.

So how exactly is change in temperature causing the change in optical density/refractive of the medium?

Thanks in advance.

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The refractive index (and hence the optical density) of a medium is a function of its permittivity and permeability which in turn are functions of temperature. This may be the reason of variation of refractive index with temperature

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    – Community Bot
    Sep 25 '21 at 16:38
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The speed of light in a medium is inversely proportional to the optical density (or refractive index) - by definition.

It is true that optical density is not directly related to 'normal' (mass over volume) density. Nonetheless it is a helpful guide. In particular glass > water > air > vacuum for both measures. (Perhaps there is some esoteric exception to this - but it's usually right.)

I'm no expert on the change in optical density with temperature, but I would guess:

  • for air, density goes down with increasing temperature and so optical density goes down.

  • for most solids and liquids, molecules bounce around more with increasing temperature and so optical density goes up.

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All commonly-occurring materials that are transparent to light have positive temperature coefficients of expansion: they swell up slightly when heated, and hence their density decreases. The density decrease causes the index of refraction to decrease in proportion.

This is true no matter what their nominal (room-temperature) density might be to start with.

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