  We generalize the notion of the Franhoufer diffraction from a single slit and
a circular aperture to the case of partially temporal coherent and
quasimonochromatic light. The problem is studied analytically and the effect of
coherence length on the diffraction pattern is investigated. In this case the
far-field distribution of the irradiance depends on the newly introduced
parameter(decoherence parameter) which governs the deviation of the diffraction
pattern from the usual one. The corrections due to temporal decoherency on the
irradiance distribution in the far field is obtained. Numerical study of the
effect of decoherence parameter on the Far-field diffraction pattern is
performed. In the case of a single slit, there is no noticeable deviation in
the central peak, but in the higher orders of diffraction, deviation become
apparent. For circular apertures, as long as the decoherence parameter is
greater than one , the beam decoherency affects the distribution profile and
the first order diffraction pattern decreases and by increasing the decoherence
parameter, the first order of diffraction pattern gradually disappears.
