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Polarization with three polarizers? Please help!

  1. Mar 29, 2012 #1
    1. The problem statement, all variables and given/known data
    A helium-neon laser emits a beam of unpolarized light thatpasses through three Polaroid filters, as shown in the figure . The intensity of the laser beam is I[itex]_{o}[/itex].
    Walker.25.72.jpg

    Suppose the third filter were at an angle of 50˚, what would be the intensity at point C?


    2. Relevant equations
    I = Io cos^2(x)

    I = Io/2 (Unpolarized light through transmission axis)


    3. The attempt at a solution
    I've tried a bunch of things....

    First I calculated the intensity at point B, which I found to be .375 Io via (1/2)Io*cos^2(30˚)

    Then I tried .375Io*cos^2(50˚)

    and also tried replacing .375 with .375/2 and just 1/2

    But I can't get my teacher's answer, which is .331 Io
     
    Last edited: Mar 29, 2012
  2. jcsd
  3. Mar 29, 2012 #2

    collinsmark

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    Gold Member

    So far so good! :approve:
    But the light just passed through polarizer 2 which was configured at 30o. That means not only the light is already polarized, but the light already has a polarization angle of 30o before it even gets to polarizer 3 (the one configured at 50o).

    So what's the angular difference between polarizer 3's angle and the polarization angle of the light at B?
    Now you're just randomly guessing. :grumpy:
     
    Last edited: Mar 29, 2012
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