A simple, single-mode, coherent Gaussian laser beam with wavefronts that are spherical as they pass through a reference plane is assumed, and then by using only the Huygen's principle and scalar diffraction theory, the propagation characteristics are found, and the formulas for the beam at a distance from the reference plane are derived. The action of a lens on a wavefront is discussed, and the lens formula is derived.
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Calum Williams (1973) studied this question.
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