= Wave focusing
<Wave focusing> brings contributions from an extended incident field into a smaller region so that their <wave phases> reinforce there. A converging lens redirects a <wavefront>; a phased source array instead sets the relative <wave phases> at emission. In either case, the field near the focus is the <superposition> of the arriving contributions.
An ideal geometrical ray description can concentrate rays to a point, but <diffraction> gives a finite focal spot for a finite aperture. For an aperture of size $D$ and focal distance $f$, its characteristic transverse scale is of order $\lambda f/D$, with a coefficient depending on aperture shape and the width convention. Focusing increases local <wave intensity> by concentrating available <power>; it does not create <energy>. Blocking part of the aperture or changing its <wave phase> changes both the focal spot and the surrounding pattern. These effects can be tested by scanning a small detector across the focal plane.
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