Huygens-Fresnel principle
= Huygens-Fresnel principle
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{wiki=Huygens–Fresnel_principle}
The <Huygens-Fresnel principle> describes propagation by coherent addition of secondary-wave contributions from a wavefront or aperture. Their <complex amplitudes>, including relative <wave phases>, must be summed before calculating <light intensity>. The <Fresnel propagator> follows by retaining quadratic propagation <wave phase> in the <paraxial approximation>.