Paraxial imaging: Gaussian beams versus paraxial-spherical waves

The features of Gaussian beam imaging an not generally well appreciated. He re we suggest that it is convenient to view. a Gaussian beam as a generaliz ation of a paraxial-spherical source with a. complex source location. This approach allows one to show that the modification of the geometric optics t hin lens equations necessary to adequately describe the imaging of a Gaussi an beam is due to the latter being a coherent source of finite extent. An e xperiment is outlined which highlights the differences between simple geome tric and Gaussian beam imaging. Our approach gives students a firm conceptu al foundation from which to discuss the concept of spatial coherence in las er beams. (C) 1999 American Association of Physics Teacher.