For an electromagnetic wave, the time over which a propagating wave may be considered coherent. Note 1: In long-distance transmission systems, the coherence time may be reduced by propagation factors such as dispersion, scattering, and diffraction. Note 2: In optical communications, coherence time, , is calculated by dividing the coherence length by the phase velocity of light in a medium; approximately given by = 2/ (c) where is the central wavelength of the source, is the spectral width of the source, and c is the velocity of light in vacuum. Note 3: "Coherence time" is usually applied to the optical regime.