A type of dispersion that occurs in optical fiber due to the interaction of various wavelengths with the physical matter in the crystalline structure of the glass. The refractive index of the glass varies according to the wavelength of the optical signal, i.e., different wavelengths travel at different speeds in the medium.The longer the wavelength, the faster the signal travels. No pulse is perfectly defined, i.e. includes just one wavelength. Rather, an optical pulse emitted by a light source has a certain spectral width, i.e., includes a range of wavelengths of lesser power around the center wavelength. The effect of material dispersion is that the various wavelengths comprising the pulse travel at different velocities through the medium. So, the pulse can spread over a distance simply due to the interaction of various wavelengths with the matter in the crystalline core, which causes some portions of a pulse to arrive earlier than other portions. As the wavelength increases (and frequency decreases), material dispersion decreases. So, optical signals in the 1550 nm window suffer less from material dispersion than wavelengths in the 1310 nm window. Material dispersion, which is synonymous with intramodal dispersion and spectral dispersion, is one factor contributing to chromatic dispersion. Material dispersion and chromatic dispersion are issues in long haul fiber optic transmission systems (FOTS) employing single-mode fiber (SMF) of step-index construction. Multimode graded-index fibers suffer so much from modal dispersion over short distances that material dispersion and chromatic dispersion never become factors. See also chromatic dispersion, dispersion, graded-index fiber, MMF, modal dispersion, refractive index, SMF, spectral width, step-index fiber, and window.