American Journal of Physics
A reflection caustic consisting of an x or a cross surrounded by a circle was observed using two different reflector geometries. Double-glazed insulating windows that have become bowed, and circularly symmetric cookie trays and cake covers possessing two different curvatures at different distances from the symmetry axis, produce similar caustics when plane waves are incident far off-axis. For the case of the insulating windows, it was found that the concavely bowed outer pane produced the x portion of the caustic and the convexly bowed inner pane produced the circle portion. For the case of the circularly symmetric reflectors, the x and circle portions of the caustic were found to be produced by the regions of greater and lesser curvature, respectively. These results were verified by numerical spot diagram calculations for each of the reflector geometries.
Lock, James A. and Woodruff, Judith R., "An Analysis of Two Unusual Reflection Caustics" (1989). Physics Faculty Publications. 68.
Lock, James A. and J.R. Woodruff. "An Analysis of Two Unusual Reflection Caustics." American Journal of Physics 57 (1989): 260-264.
Copyright 1989 American Association of Physics Teachers. The article appeared in American Journal of Physics 57 (1989): 260-264. http://aapt.scitation.org/doi/pdf/10.1119/1.16049