Model a parabolic graded-index fibre with n (r) = n ₁ [1 - ( r / a) ² ] for r  ≤ a , with n ₁ = 1.5 , a = 0.1 mm and Δ = 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 5 degrees 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?