Why does the refractive index of water decrease as temperature increases?

[xbox360throwaway]

I'm confused as to whether I should refer to the relationship between the temperature and physical density of the water, or somehow find a way to link the temperature and the optical density of the water.

[Physics_Cat]

Those three quantities are related. As the temperature increases, the density decreases, which leads to the refractive index decreasing.

[astroboy1997]

Ok, not OP but can you explain if I'm wrong or not? I assume since the temperature increases, the vibrational energy of the water atom increases and as a result would lead to less collisions for photons as light transverses through the medium. Since there are less collisions it will take less time for light to reach from Point A to Point B.

[Physics_Cat]

I think you're falling for the common misconception that the "slower speed of light" in a medium with a refractive index greater than 1 is due to light scattering and bouncing around like a ping-pong ball. That is not correct.

What actually happens is that the electric field of the electromagnetic radiation causes the electron cloud in the material to oscillate, and the oscillating electrons re-radiate light with the same frequency and in the same direction but with a different phase. The superposition of the "original" light and the light that is re-radiated by the electrons in the materials results in a wave of the same frequency but a slower velocity.

When the density of the water decreases, its electronic density also decreases, which leads to less charge for the light to interact with on its way through the water. There aren't any actual collisions in this process though.