Refractive index mysteries of water

Even though the refractive index is dominated by single molecule effects, s uch as the polarizability, there are some aspects of this property for wate r that are different from the refractive index behavior of most other liqui ds. Here, we discuss, then explain, these differences in terms of a two-sta te bonding model for this liquid. A simple two-parameter formulation based on this model is found capable of describing the refractive index of water, including its thermal maximum near 0 degrees C, from slightly supercooled temperatures to about +45 degrees C within a four- to five-decimal place ac curacy. The agreement with experimental data achieved here using this simpl est of possible models provides further strong support for the two-state de scription of this liquid.