Dispersion curves are important when choosing materials for the optical components of instruments. A substance that exhibits normal dispersion over the wavelength region of interest is most suitable for the manufacture of lenses, for which a high and relatively constant refractive index is desirable. Chromatic aberrations (formation of colored images) are minimized through the choice of such a material. In contrast, a sub-stance with a refractive index that is not only large but also highly frequency dependent is selected for the fabrication of prisms. The applicable wavelength region for the prism thus approaches the anomalous dispersion region for the material from which it is fabricated.