The current alternatives regarding materials and designs of ophthalmic lenses makes difficult to decide the ideal lens for a specific prescription, of course if optical and physical properties are not known in depth. This article describes how the refraction index of a material depends on the light wave longitude through electrical polarization, producing the dispersion process. In ophthalmic lenses, chromatic dispersion is characterized by the Abbe number. Some definitions for this number, its importance and how to minimize its effects in ophthalmic lenses are also shown.