The features of Gaussian beam imaging an not generally well appreciated. Here we suggest that it is convenient to view. a Gaussian beam as a generalization of a paraxial-spherical source with a. complex source location. This approach allows one to show that the modification of the geometric optics thin lens equations necessary to adequately describe the imaging of a Gaussian beam is due to the latter being a coherent source of finite extent. An experiment is outlined which highlights the differences between simple geometric and Gaussian beam imaging. Our approach gives students a firm conceptual foundation from which to discuss the concept of spatial coherence in laser beams. (C) 1999 American Association of Physics Teacher. [References: 8]
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