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When a wave with a lengthwave of the same dimension of a slit passes through a slit, each point of the slit behaves as a source of secondary waves. These secondary waves interfere each other.

If consider the Bragg's diffraction:

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Why the points between two consecutive atoms do not behave as source of secondary waves when the incident (and diffracted) beam 2 (2') passes through?

Thank you for your help.

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Also in screens and slits, it is the atoms in the material that radiate. Babinet's principle ensures that one gets the same diffraction patterns from a line as from a slit. And realistically, the pattern from i slit is caused by the rest of the screen.

Water waves in a ripple tank are mostly an analogy. Huygen's principle is just a model. The vacuum does not really scatter EM waves.

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  • $\begingroup$ Hi @Pieter. I read on Wiki that the diffraction pattern from an opaque body is identical to that from a hole of the same size and shape except for the overall forward beam intensity (Babinet's principle). I don't understand why to a single incident beam correspond a single diffracted beam (elastic scattering). The interference exist if there is superposition of waves; in this case I see only a single diffracted beam instead of multiple beams which interfere. $\endgroup$ – Gennaro Arguzzi Dec 9 '16 at 8:08
  • $\begingroup$ @GennaroArguzzi You are right, a single atom scatters in all directions. The diagram is a shortcut. The scattering from a plane is a sum over all the scatterers in that plane. These add in phase in the direction given by the condition for specular reflection. That is represented by just one reflected wave. It is am essential step, often not explicitly made. $\endgroup$ – user137289 Dec 9 '16 at 9:39

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