I have to find the first minima for a single slit diffraction problem. I know the wave lenght and the slit size, there is an unknow angle between the incidence wave and the perpendicular line to the slit plane. I konw that i have to use the Fraunhofer euqtion for a single slit, i already have the correct answer. But I must to explain why the incident angle does not affect the result. The equation applies for a monochromatich wave, wich front is parallel to the slit, and the wave lenght<<slit size. right? If the wave front is parallel to the plane is difference in the phase of the secundary sources, ok? ( I´m not sure of this) If there is an angle this phase difference is another, but the Fraunhofer equation gives us the answer anyway?