Fast measurement of the refractive-index dispersion of dilute liquid solutions of absorbing molecules

We describe a fast method for measuring the refractive-index dispersion of transparent and absorbing liquid solutions. The method is based on measuring the reflectance spectra of an optical beam of white light in an internal reflection configuration near the critical angle defined by the refractive indices of the incident medium (a glass prism) and that of the solvent, which is assumed to be transparent. From a few reflectance spectra taken near the critical angle with the pure solvent and the solution, the dispersion of the contribution to the refractive index of the solute (the sample) can be obtained. We present results with solutions of Rodamine 6G dissolved in distilled water and in methanol showing the feasibility of the proposed method.