How and why does the Huygens principle really work? I mean, does it always work?
The Huygens principle:
Every point on a wave-front may be considered a source of secondary spherical wavelets which spread out in the forward direction at the speed of light. The new wave-front is the tangential surface to all of these secondary wavelets.
For the Huygens principle to be true, light must emit light. Can light emit light?
Answer
The key phrase is "may be considered". That is, Huygens principle, as a mathematical procedure, gives (approximately) the right answer. The principle does not say that light is actually being generated at each point on the wavefront.
Another way to look at it: the light at a phase front already exists there. There is no need to generate it all over again.
It's interesting to note that if the vacuum were a "perfect" polarizable material, then the incident light would polarize every point in space, and each point would scatter light into spherical waves. I don't think that there's any physical justification for the picture, and it might lead to dangerously wrong conclusions, but it might help give you a grasp of how the mathematics works.
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