Purpose:Ilustrate the location of the principal planes in a telephoto lens.
Description:Form a "telephoto lens" from the combination of one double-convex lens and two plano-concave lenses (forming one double concave lens), separated by ten inches center-to-center. By producing parallel rays with the convex lens at the left, one can locate the focus and trace rays back to locate the principal plane. Reverse the "telephoto lens" elements to locate the other principal plane. For comparison with the measurement, the foci and principal planes can be calculated using f(c)=20", f(d)=-20", and t=10". Choice of slit baffle and distance of baffle from the light source determine the number of rays and their spacing.