Basic optics texts have been using the phrase dispersion both in the context of material dispersion due to the frequency-dependent velocity of light in different materials and the spectral dispersion of optical frequencies out of an incident beam into its component frequencies. Sometimes this dispersion is achieved by exploiting both the physical processes as in prism spectrometer. This chapter analyses the physics behind these two dispersion processes and then elaborate on the roots behind material dispersion. It shows that the mathematical Fourier frequencies due to a pulse envelopes being nonphysical, their propagation can give nonphysical results. The chapter explains the emergence of spectrometric dispersions with reference to the resolving powers for three representative spectrometers, which are in use in most laboratories: prism spectrometers, Fabry–Perot spectrometers, and grating spectrometers. It discusses their comparative relevancy and contrast with prism spectrometers.