A sequence of equally space identical lenses can form a "lens waveguide" as noted in class. Here, each "cell" of this periodic structure consists of free-space propagation through a distance d followed by a thin lens of focal length f. a) Find the condition for stability for this configuration. b) Sketch the ray path through six "cells" of this structure for Case I: d = 2f; and Case II: d = f, when the initial input ray is parallel to the optic axis.