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 and focal distance , its characteristic transverse scale is of order , 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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