The refractive index of water has a maximum just below 0 degrees Celsius

Scientific Explanation

The refractive index describes how strongly a medium bends light rays. In most liquids, the refractive index decreases monotonically with rising temperature as the density drops and molecules move farther apart. Water behaves differently: its refractive index rises as temperature falls, reaching a maximum just below 0 degrees Celsius before declining slightly at even lower temperatures in the supercooled state.

This maximum is closely connected to the density anomaly. The refractive index is linked to density through the Lorentz-Lorenz equation: higher density generally means a higher refractive index. Since water’s density peaks at 3.98 degrees Celsius, one might expect the refractive-index maximum nearby. It actually lies somewhat lower (near 0 degrees) because molecular polarizability also plays a role and changes with the restructuring of the hydrogen-bond network.

In the supercooled region, the density decreases again as the open, ice-like structure prevails, and the refractive index drops with it — hence a maximum rather than a monotonic rise.

Everyday Relevance

The refractive index affects how we perceive water optically. The slight change with temperature creates schlieren patterns in natural bodies of water when water of different temperatures meets. In analytical chemistry, refractometers based on precise refractive-index measurements are used to determine the purity and concentration of aqueous solutions — and the temperature dependence of water’s refractive index must be taken into account.