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I was hoping to ask a question about Huygens' principle, which I am using to understand diffraction and reflection. I have seen videos and websites like these that explain why Huygen's principle leads to the law of reflection: https://www.youtube.com/watch?v=N3levs4TzTA https://www.compadre.org/nexusph/course/How_Huygens_gets_reflection_and_refraction_(technical)

These resources appear to examine isolated points on the reflective surface, so I will do that too below.

I am struggling with one thing. Huygens' principle says that the wavefront can be modeled as a bunch of spherical wavelets. According to the resources, when the wavefront encounters a reflective point, the point begins to emanate wavelets. enter image description here

However, supposing this reflective point did not exist, it seems like wavelets would already have been emanating from that location, as wavelets emanate from all points on the wavefront. I thought about if there was a difference in phase from the wavelets generated from the reflective point, and the wavelets at the wavefront, but they appear to have the same phase.

In Huygens' model, then, is the reflective point just treated as a 'point source' of wavelets, whose phase is the same as the incident wave?

If this is the case, it seems like this reflective point is just the opposite of an 'absorptive' point. The reflective point increases the energy of the wavelets that it touches on the wavefront, while the absorptive point decreases the energy of the wavelets that it touches on the wavefront.

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  • $\begingroup$ Note that Huygen's principle is a heuristics. The true source of waves is the physical thing making the waves, and this wavefront method is but just a mathematical trickery to help us visualise what is going to happen. For example of its inconsistency, you only draw half of the wave, the part propagating outwards, and not the part going back to the source. This is where the reflecting point does differently, it makes spherical waves going backwards, which is definitely different from the rest of the wavefront. I don't remember what it does to the phase. $\endgroup$ Commented May 8, 2023 at 5:53

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"In Huygens' model, then, is the reflective point just treated as a 'point source' of wavelets, whose phase is the same as the incident wave?" YES

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