I have a doubt regarding diffraction in single slit. Diffraction is nothing but the interference of secondary wave fronts. As secondary wave fronts originate from primary wave fronts similarly a teritiary wave front must originate from the secondary wave fronts. And this process continues until the intensity of the wavefronts is zero. Then why don't we consider the interference of teritiary wavefronts or quarternary wavefronts so on?
It's a boundary value problem-- to understand the waves on the wall, you only need to understand the waves on the boundary that is the slit. There is no amplitude contribution at the wall that is not represented at the slit, so you account for all the tertiary stuff by looking at what is happening at the slit.