A non-interferometric differential phase measuring technique is used to determine the refractive index of a medium. In total internal reflection (TIR), phase change is a function of the refractive indices of a pair of media involved. Also, the said phase change is different for p-polarized and s-polarized light components. Difference between the phases of these two polarized light components leads to the refractive index of the sample. The theory of the technique is discussed and a simple experimental method is demonstrated to determine the phase difference of the totally internally reflected p and s components. © Springer Nature Singapore Pte Ltd. 2017.