I've already seen some answers here: Why are diffracted wave fronts drawn with curved edges in textbook diagrams, when according to Huygen's principle they should be straight lines? and from what I've understood, with Huygens' original principle, you would have had just a straight wavefront propagating after the slit, as shown by the images in the post. Instead, with the Huygens-Fresnel principle, we have a curved wavefront near the edges of the obstacle. Why exactly do Fresnel's changes to Huygens' principle allow for this curved wavefront? Why doesn't a plane wave have that curve at its edges too?

  • $\begingroup$ "Why doesn't a plane wave have that curve at its edges too?" — plane wave doesn't have edges: it's extended to infinity $\endgroup$ – Ruslan Feb 18 at 9:11
  • $\begingroup$ if it's extended to infinity then why isn't the wave itself infinitely large? $\endgroup$ – Kouta Dagnino Feb 18 at 13:32
  • $\begingroup$ It is, why do you think it isn't? Plane wave is an ideal object, you'll never get one in real life. Or are you talking about the amplitude instead of spatial extent? $\endgroup$ – Ruslan Feb 18 at 13:34
  • $\begingroup$ nono i was thinking about spatial extent. So to sumarize, the wavefront is the envelope of the secondary spherical wavelets produced by the wavefront. Since this envelope is curved in the case of diffracted waves, the wavefront gets curved at the edges, whereas in a plane wave, since you have an infinitely far away source (and thus extends to infinity), the envelope must be straight? $\endgroup$ – Kouta Dagnino Feb 18 at 14:02

Look at the introduction in Wikipedia:

The Huygens–Fresnel principle (named after Dutch physicist Christiaan Huygens and French physicist Augustin-Jean Fresnel) is a method of analysis applied to problems of wave propagation both in the far-field limit and in near-field diffraction and also reflection. It states that every point on a wavefront is itself the source of spherical wavelets, and the secondary wavelets emanating from different points mutually interfere. The sum of these spherical wavelets forms the wavefront.

(bold mine)

The difference is that in the original Huygens principle

He assumed that the secondary waves traveled only in the "forward" direction and it is not explained in the theory why this is the case

The mathematical form of the secondary wavelets was used correctly by the addition of Fresnel, so the phases were kept and thus diffraction can happen.

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  • $\begingroup$ But why does fresnel's new principle make the curves at the edges of the wavefront? $\endgroup$ – Kouta Dagnino Feb 18 at 13:14

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