Fraunhofer's method used to measure the refractive index of liquids has been modified. The modification includes an electric circuit containing a sensitive photodiode in order to measure accurately the deviation angle of a HeNe laser beam. The refractive index of pure water and four different concentrations of NaCl solution is measured at temperature range 5-90°. Four mathematical equations relating the refractive index to the temperature and concentration are obtained using the least squares method. The effect of temperature and concentration on refractive index are explained using Lorentz-Lorenz formula.

K. M. Aly, E. Esmail, "Refractive index of salt water: effect of temperature," Opt. Mater.,2, 195-199 (1993).