the thick lens shown in 5 is exposed to parallel light
(1)treat the lens as thin with the location of the thin lens at the center of the actual lens and in this way calculate the focal point for the rays
(2)repeat this exercise treating the lens as thick yet still in the par axial limit.
(3)use the exact ray-tracing formalism to find the optical axis intercept of rays that are 0.25 in. and 0.5in. from the optical axis on the incident side.