  We have analytically explored the Rayleigh-Sommerfeld scalar diffraction for
oblique incidence. We have explicitly derived the Fraunhofer diffraction
formulae for oblique incidence of plane scalar wave on various apertures, such
as single-slit, circular aperture, and diffraction grating. Such derivations in
the background of Rayleigh-Sommerfeld scalar diffraction theory would be
important for an undergraduate course on optics.
