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Did I get this right? Matrix system of a lens

  1. Oct 20, 2015 #1
    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.
  2. jcsd
  3. Oct 22, 2015 #2


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    Science Advisor
    Homework Helper

    You made a mistake here. The free propagation should come first, then followed by the lens.
  4. Oct 23, 2015 #3
    Oh okay, thank you very much for your help!
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