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Homework Help: Polarizers in series, I got the correct answer, want to verify why

  1. May 17, 2013 #1
    1. The problem statement, all variables and given/known data
    An unpolarized laser emits 1.00 W of continuous optical power.
    It's beam is directed into three identical linear polarizers, each with a maximum transmission of 95.0%. The first polarizer has its transmission axis vertical, the second at 45.0 from the vertical, and the third is in an unknown orientation. An optical power meter reads the overall power transmitted as 5.50 milliwatts. Find the orientation of the third polarizer's transmission axis relative to the vertical. (20 points)

    2. Relevant equations

    I = Imax*1/2 * cos212)

    3. The attempt at a solution

    So I set up the equation as such:
    5.5*10-3= (0.95)3*1.0W*1/2 * cos2(90°-45°)*cos2(θ - 45°)

    I do some algebra and get θ = 125.78, which is the correct answer.

    What I'm looking to clarify is a few things.

    For the (0.95)3 part of the equation, the exponent will always just be the number of polarisers if they all have the same maximum transmission?

    cos2(θ - 45°), the -45° comes from the previous polariser right? So if there was another polariser that had an angle 60° from the vertical place before the last unknown polariser, the equation would change to:
    5.5*10-3= (0.95)4*1.0W*1/2 * cos2(45°)*cos2(60°-45°)*cos2(θ - (60°-45°))

    Is this correct? I'm not 100% certain and I don't have access to any more problems in the same form as this one that I have the answer to.

    Thank you in advance.
  2. jcsd
  3. May 17, 2013 #2


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    2017 Award

    Staff: Mentor

    I think the angles should be 45°, 15° (=60-45) and θ-60°, as θ is measured relative to the vertical axis.
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