I don't understand how the formula works; vD= nD-1 / nF-nC given nD,nF and nC are the Fraunhofer's line with wavelength of 589.3, 486.13, and 656.27 respectively in nm. Now my trouble starts here. Since the wavelengths are already given , VD is always a constant. How does the relationship between wavelength and refractive index goes here? Say if I want to measure a random value of refractive index/wavelength of a glass, how do I put the refractive index of my glass into the above equation?