Analogy with mechanics to introduce optical imaging and to teach astigmatism

A simple analogy between mechanics and optics has been found for introducing imaging proprieties of lenses. In this analogy the light beam in optics has the role of a sample beam in mechanics; while in optics the light beam is expanded or reduced in size by the power of the lens in mechanics the sample beam suffer elongation when a tensile or compressive stress is applied. Following the analogy the aberration introduced by the lens can be the corespondent non linear effect that occurs in mechanics when elastic limit is over. Using the simple developed analogy astigmatism aberration of cylindrical lenses can be explained trough a very simple classroom demonstration using robber stripes.