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This question does not concern a homework problem but I don't really understand the Huygens-Fresnel principle of diffraction. My book states that an assumption is made that a wavefront acts as a source of secondary wavelets. They continue with the following derivation. This derivation concerns a single slit experiment with a source point and an observation point at either side of the slit.

Let ##r'## be the distance between the source point and the slit. The wave will reach the slit at ##t=0## so the amplitude becomes $$E_A = \frac {E_0} {r'} e^{ i(kr')} $$ Now let the distance from the slit to the observation point equal ##r##. They state the amplitude at this observation point caused by a single wavelet will then equal $$dE_p = \frac {E_A} {r} e^{ i(kr - w*t)}$$ This is where I get confused. Why does the source amplitude of such a wavelet equal the amplitude of the entire original wave? The way I understand it the number of wavelets should go to infinity. so how come they all have the amplitude of the original wave. Wouldn't you be creating energy out of nowhere?

Thanks!

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# I Huygens-Fresnel principle of diffraction

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