In this work, we explore the refractive index of the phasmidic columnar liquid crystal for homeotropic oriented samples (column perpendicular to the plates). To measure the ordinary index n(o) of the columnar liquid crystal C-10, a simple and accurate method is presented using the deflection by reflection of a laser beam (lambda= 633nm) through a prismatic phasmidic thin film placed between two glass plates in the homeotropic orientation. For the C-10 samples, we found n(o) = 1.5512 +/- 0.0002 at T = 100 degrees C. We used the same method to determine the isotropic index n(l) of the isotropic phase and found n(l) = 1.5301 +/- 0.0002 at T = 120 degrees C. Using the precision of the same method in polychromatic laser beam, we measured the variation of the ordinary index with the wavelength and characterize the dispersion by the known Cauchy's dispersion relationship n(o) = A + B/lambda(2) We deduce an estimation of the birefringence for several samples.