def sourceSingleSlitOpaqueWall(a, da):
    """ We return two lists: one with the space coordinates, one with the 
    amplitude at the corresponding coordinate. 
    Amplitude is zero outside of coordinates from aCoords. """

    Na = int(a/da)
    amplitude = (1/float(Na)) # normalized

    aCoords = [(j-Na/2)*da for j in range(Na+1)] # space coordinates
    amplitudes = [ amplitude for y in aCoords] # constant amplitude

    return (aCoords, amplitudes)