Which principle uses refractive index (RI) to characterize substances?

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Measuring how light bends when it enters a material—its refractive index—provides a unique signature of that substance. The refractive index tells you how much light slows and changes direction inside a sample, and this value depends on the material’s composition, concentration, and purity. A refractometer is specifically built to determine this index by comparing the path of light in air (or a reference) to its path in the sample, giving a direct RI measurement that can identify or quantify the substance.

That direct link to refractive index is why the refractometer is the best choice here: it is designed to characterize substances precisely by their RI. In contrast, a fluoridometer measures fluoride ion concentration, a turbidometer assesses how cloudy a solution is (turbidity) due to suspended particles, and a spectrophotometer analyzes how much light is absorbed at specific wavelengths. None of those rely on refractive index as the primary basis for characterization.