We have used a laser interferometric method to determine the refractive index of some laser dye solvents with a high accuracy of 5.8×10^{-4}. The determination of refractive index (n) has been done by counting the interference fringes (N) as a function of the incidence angle of the laser beam. The absolute refractive index of pure water, dimethylsulfoxide, and different mixtures of percentages (P) 25%, 50%, 75% of dimethylsulfoxide has been measured. The relation between the bulk refractive index (n_{m}) of these mixtures is plotted as a function of the investigated percentages. The temperature behavior of these refractive indices (dn_{m}/dt) was studied carefully within a temperature range of (20-80°C). We have found that both the refractive index (n_{m}) and temperature behavior of the refractive indices (dn_{m}/dt) of different mixtures of the investigated solvents have the average index properties of the parent solvents. Also, a fitted empirical relation between n_{m}, P, and t has been assumed as n_{m} =aP^{b}t_{c} with empirical constants a, b, and c. According to that equation we have recalculated n_{m} of these solvent mixtures and we have found satisfying agreement with the experimental results.
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