The spreading of a pulse in an optical fiber caused by differences in wave velocity in the medium, chromatic dispersion is measured in picoseconds of pulse spreading per nanometer of spectral width per kilometer of fiber length. Chromatic dispersion is the sum of waveguide dispersion and material dispersion. Material dispersion is caused by the fact that the speed of light in a medium is sensitive to the wavelength, i.e., the velocity of light in a medium depends on its wavelength. Waveguide dispersion is caused by the fact that a given wavelength travels at different speeds in the core and cladding of a single-mode fiber (SMF). Material dispersion, waveguide dispersion, and, therefore, chromatic dispersion, are issues in long haul fiber optic transmission systems (FOTS) employing single-mode fiber (SMF) of step-index construction. Multimode fiber (MMF) and graded-index fiber suffer so much from modal dispersion over short distances that material dispersion and chromatic dispersion never become factors. Chromatic dispersion is so named as different wavelengths of light in the visible spectrum display as different colors. See also graded-index fiber, material dispersion, MMF, SMF, step-index fiber, and waveguide dispersion.