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Homework Help: Number of reflections undergone by light ray between 2 plane mirrors

  1. Jun 6, 2006 #1
    Two plane mirrors M1 and M2 have a length of 2 meters each and are 10 cm apart. A ray of light is incident on one end of mirror M2 at an angle of 30 degrees. What is the number of reflections light undergoes before reaching the other end?
    I solved it in the following way:
    In each reflection the distance moved = 10 x tan(30)
    = 5.77 cm
    Therefore the number of reflections before it reaches
    the other end = 200/5.77
    = 34
    But the answer given in my book is 170. Please help.

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  2. jcsd
  3. Jun 6, 2006 #2

    Doc Al

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    Staff: Mentor

    Your answer and method make sense to me; the book's answer does not.
  4. Jun 6, 2006 #3


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    Staff Emeritus
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    Gold Member

    I would also agree with you here. I obtained answer identical to yours Amith (34.64...) which rounds to 34 reflections. It seems your 'textbook' is making a significant number of errors. If you don't mind me asking what text are you reading from?
  5. Jun 6, 2006 #4
    I kindly request you all to tolerate the erroneous questions of my book. There are about 150 questions in each unit out of which say about 7 to 8 questions have wrong answers. That is why I am still using it. I hope to get it corrected with your help.Thanks for your guidance.
    Last edited: Jun 6, 2006
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