(6) (20 pts) A glass lens in air has n = 1.5, R12 = 200 cm, and R23 = -200 cm. (a) Assume it is a thin lens and find its focal length. (b) Now assume that the lens has a thickness t = 5 mm. Find its effective and back focal lengths. (c) Will the rays cross the axis first, i.e., focus first, for the lens described in (a) or (b)