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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?

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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.

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  • $\begingroup$ Or, put differently for the OP, the separation you are attempting into primary, secondary, tertiary, ... is not found in the real math - the wave function propagates from the slit to the screen, interfering with itself. What is seen on the screen is not from 'secondary' waves - it is all one wave obeying the same wave equation, but by imposing a slit you force the end result of the wave propagation to be the interference pattern. $\endgroup$ – Jon Custer Oct 18 '16 at 13:43
  • $\begingroup$ Yes, solving the wave equation has the same net result of adding all the different ways amplitudes can propagate from sources to the screen, as per a kind of Feynman path integral. When one is doing a path integral, each step is just like there was a source at the previous step, but solving the wave equation takes into account all of that. $\endgroup$ – Ken G Oct 18 '16 at 15:26

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