I have recently started learning about physical or wave optics and one of the initial topics is Huygens' Principle.

One part of Huygens' Principle states that every point on the wavefront acts as a source of secondary spherical wavefronts

Now I have two questions:

  1. Why is that? Why is every particle producing its own wavefronts? Can someone explain what exactly happens at the particle level to beget this phenomenon?

  2. What about the points on the secondary wavefronts? Are they producing their own wavefronts too? If yes then the cycle would never stop and this seems so unfathomable to me.

I would appreciate as an answer that doesn't revolve around pure math but is rather more intuitive.

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    $\begingroup$ No, don't ascribe more legitimacy to Huygen's principle than necessary. It is just a mathematical trick for finding where the next wavefront will be. The source of the wave is the thing that sets everything going. There is no particle level behaviour that is causing the Huygen's principle to be true. $\endgroup$ Commented Jan 22 at 13:53
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    $\begingroup$ Huygen's principle is interesting in that it kind of pre-dates the idea behind the path integral of quantum mechanics. It is incomplete because it uses classical amplitudes and it is a scalar model, so it couldn't reproduce solutions to Maxwell's equations even if it were the correct approach. If you look at water waves scattering on small objects floating on the surface you can kind of see where it might come from. $\endgroup$ Commented Jan 22 at 14:04
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    $\begingroup$ Huygens' intuition came from his experiments. It doesn't seem to have been intuitive to anybody else before Young's diffraction experiments. So, if you want to build that intuition, play around with lenses, gratings, antennas, etc. $\endgroup$
    – John Doty
    Commented Jan 22 at 14:07
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    $\begingroup$ I talk about this in my answer to Explanation of diffraction of a single light ray by Huygens' principle $\endgroup$
    – mmesser314
    Commented Jan 22 at 15:14
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    $\begingroup$ There are no physical waves or particle-level description, it's just a way to visualise whatever is happening. $\endgroup$
    – Aman pawar
    Commented Jan 22 at 15:35

1 Answer 1


A round object that is pushed into the water causes a uniform displacement of the homogeneous medium around it and a ring-shaped wave is created.

If I replace this one object with a sequence of objects - a ruler for example - the homogeneity of the medium along the ruler is disturbed. The medium can only move in front of and behind it, forming a plane wave - except at the ends of the ruler. There, the ends cause ring-shaped waves again.


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