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I have a few questions regarding Huygens' principle. How does the concept of wavelets make sense? I don't get how waves can create a new wave, and what is the need to create those waves when they are already present there?

Suppose I have a wavefront. My book says each point on it acts as a source of disturbance and create wavelets, and the common tangent enveloping them is a new wavefront. Now my doubt is: suppose a new wavelet is just created; then this wavelet should immediately create a new wavelet and now this should act again as a source and so on. But in my books and other resources I find only one wavelet from each point on the wavefront expanding till a new wavefront is formed. Why is it so?

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    $\begingroup$ Does this answer your question? Huygens-principle $\endgroup$ Jan 18, 2020 at 19:58
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    $\begingroup$ I've always viewed Huygen's principle as more of a mathematical one than a physical one. $\endgroup$ Jan 18, 2020 at 19:59
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    $\begingroup$ Only one wavelet is shown because it's impossible to render onto paper the scenario you describe. The paper would be black! It's really not right to say that each point is a source. There's nothing there that can be a source. Better to say that the field behaves as if there were a source at every point $\endgroup$
    – garyp
    Jan 18, 2020 at 20:31

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I find only one wavelet from each point on wavefront expanding till a new wavefront is formed. Why is it so?

The figures showing a wavelet from each point on wavefront are taken at a single point in time, say $t_0$, so only one set of wavelets are shown--all with the same radii equal to $ct_0$ (c is the propagation velocity). Analyzing the same wavefront at a later time, say $t_1$, would show the same wavelets but with larger radii, all equal to $ct_1$.

If the analysis is for an initial wavefront at a later time the same steps are followed.

All of this can be done because wave propagation is a linear process. It is a very common way to analyze linear processes--'divide and conquer'. Eg. Green functions, impulse response, ...

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