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Homework Help: Problem on Geometrical Optics

  1. Sep 2, 2013 #1
    1. The problem statement, all variables and given/known data
    A man stands 1.85m in front of a mirror hanging on a wall. His eyes are 10 cm below the top of his head. At what height from the floor, must the mirror be placed?

    2. Relevant equations

    Snells Law: n1*sin(omega)1=n2*sin(omega)2

    3. The attempt at a solution

    I converted everything to cm so 1.85= 185 cm
    and 185-10 = 175 which is the height from his eyes downward
    I just do not understand how to set it up where I could possible find the height of the mirror!
    Last edited: Sep 2, 2013
  2. jcsd
  3. Sep 2, 2013 #2

    Simon Bridge

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    Welcome to PF;
    The problem statement is incomplete - how would the man know if the mirror was the right height or not?
    "At what height from the floor, must the mirror be placed".... to do what?

    i.e. Usually the idea is that the Man has to be able to see his entire body in the mirror - in which case we also need to know the height of the mirror. If it is a full-length mirror, then it can sit on the floor. Could you work out the minimum size of mirror that would let the man see his entire body at that distance?
  4. Sep 2, 2013 #3
    Well Question a) was: What is the shortest mirror which without being moved, will permit the man to see his entire body? I solved this knowing that to see a full height of a person, he needs a mirror which has a length of half his height, which is 185/2 = 92.5 But I don't understand what does this have to do the question b)? That all they give me
  5. Sep 2, 2013 #4

    Simon Bridge

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    Well - if the mirror was sitting on the floor - would he be able to see his whole body? Or just his feet?
    What about if the bottom of the mirror were level with his eyes?
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