Refractive index measurement of absorbing and turbid fluids by reflection near the critical angle

A method is described for measuring the specular optical reflectance R(phi) of the interface between a fluid and a glass prism, the incident and reflected light being in the glass, and the angle of incidence phi being varied about the critical angle. Transparent, absorbing and some turbid fluids give R(phi) close to theoretical predictions, and experimental reflectance data are analysed to give the real (n') and imaginary (n'') refractive indices of the fluid. Other turbid fluids gave R(phi) data that differed strongly from theoretical expectation. This is attributed to heterogeneity of the fluid on the optical wavelength scale, and problems of refractometry for such fluids are discussed.