Refractive Index Mysteries of Water

Even though the refractive index is dominated by single molecule effects, such as the polarizability, there are some aspects of this property for water that are different from the refractive index behavior of most other liquids. Here, we discuss, then explain, these differences in terms of a two-state 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 °C, from slightly supercooled temperatures to about +45 °C within a four- to five-decimal place accuracy. The agreement with experimental data achieved here using this simplest of possible models provides further strong support for the two-state description of this liquid.