The extended Cauchy model is derived based on the experimental data for describing the mass fraction-, crystallite size-, and calcination-dependent refractive index of anatase and rutile TiO2 nanoparticles. This model fits the experimental data of TiO2 nanoparticles well in the visible spectral region. There is a tunable correlation between the refractive index of TiO2 nanoparticle and the anatase nanoparticle size at visible region. Moreover, there is a near correlation between experimental and calculated Abbe number at 588 nm.
