Calculate the amplitude a of an elementary secondary Huygens-Fresnel wave.
(The amplitude a is proportional to the amplitude A of the primary wave that
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came from the element ∆S, the area of this element, and is inversely proportional to r, i.e. a = cA∆S/r. To determine c, compare the plane wave amplitude at some point and the Fresnel amplitude at the same point, when the plane wave front is taken as an auxiliary surface.)