I learned about the refractive index of gases and dense materials, which is a function of the atom number per volume, wavelength of electromagnetic wave, etc. However, what is the refractive of solutions that contains ions? Is it the same as conductors or dielectrics and why? In my point of view, the mass of ions are much larger than electrons and we can neglect their vibration induced by electromagnetic wave. So we can treat the solution the same way as those without ions (such as sucrose solution), am I right?