1. The problem statement, all variables and given/known data A thin lens is placed 2m after the beam waist. The lens has f = 200mm. Find the appropriate system matrix. This is a past exam question I want to check I got right. 2. Relevant equations For some straight section [[1 , d],[0 , 1]] and for a thin lens [[1 , 0],[-1/f , 1]] 3. The attempt at a solution [[r2],[r2']] = [[1 , d],[0 , 1]] [[1 , 0],[-1/f , 1]] [[r1],[r1']] I multiplied the two matrices and found [[1-d/f, d] , [-1/f , 1]] Is this correct? Or am I meant to add an additional straight section after the lens? I get confused as to when to use the straight section matrix. Thanks.