electronics -- optoelectronics

Model a parabolic graded-index fibre with n(r )=n1 [1- (r /a )^2 (Del) ] for r <a, with n1=1.5, a=0.1mm and Del=0.01, by the equivalent multiple-step approximation. Divide the radius into 10 equal parts. Consider a ray inside the fibre crossing the fibre axis at 4.5 degree with respect to that axis. Using the graph paper, plot the progress of the ray through the fibre until it turns back and re-crosses the fibre axis. (Angles can be magnified on the plot.) At what value of r / a will the ray turn back? Does it depend on the value of a ?