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Vertically polarized light from a hene laser passes through a linear

  1. Apr 13, 2014 #1
    1. The problem statement, all variables and given/known data

    1. Vertically polarized light from a helium neon laser passes through a linear polarizer with its axis of polarization oriented 15° from the vertical axis. Assuming no absorption or reflection:
    (a) What percentage of the light will be transmitted?
    (b) What will be the polarization angle of the transmitted light?

    2. In question #1, if a second polarizer is placed in the beam after the first polarizer with its axis oriented at 45° from the vertical axis:
    (a) How much light is transmitted?
    (b) What will be the polarization angle of the transmitted light?


    2. Relevant equations
    I believe that Malus's Law applies for Q1: I = Iocos^2 [itex]\theta[/itex]i


    3. The attempt at a solution
    Q1(a+b): I = Io cos^2 (15°) = Io 0.9330 = 93.3% Io at 15°

    Q2(a): Second polarizer uses same formula except that only 93.3% of incident light has gotten through resulting in following change:
    I = .933 Io cos^2 (45°) = .933 (0.5) Io = 0.4665 Io at 30° (45-15)

    I don't have a lot of confidence in the answer to Q2.
     
  2. jcsd
  3. Apr 13, 2014 #2

    mfb

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    What is the angle between the light and the polarizer for Q2? It is not 45° - this is the angle between polarizer orientation and vertical axis.

    That is not the polarization direction after the polarizer.
     
  4. Apr 13, 2014 #3
    If the light passes through the first polarizer at 15° and then passes through another polarizer at 45° to vertical, then is the effect cumulative? Would it then be 45° + 15° = 60° overall?
     
  5. Apr 13, 2014 #4
    OK....I think I figured out some of this:
    A2: (a) I think the formula for both polarizers is: I = Io cos^2(15°) Io cos^2 (30°) which ends up to be I = 0.6998 Io or 69.98% (rounded)
    A2: (b) Still not quite sure how to figure out the polarization angle mathematically other than I'm pretty sure that the 45° polarizer results in quarter phase shift.
     
  6. Apr 13, 2014 #5

    mfb

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    Right.
    You do this way too complicated. It is a polarizer. What does a polarizer do?
     
  7. Apr 14, 2014 #6
    Thank you for your help!

    A polarizer, as I understand it, blocks/absorbs some light waves and orients the remaining light waves linearly (on a plane, actually, that diverges from the y-axis by θ) as it passes through the "picket fence" openings of the polarizer. If you have 2 polarizers oriented at 90° though, doesn't it block all light (back to Malus's Law-cos 90° = 0)?

    Does that mean, in this example, that the resultant light after passing through the two polarizers is oriented at 45°?
     
  8. Apr 14, 2014 #7

    mfb

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    Yes to both.
     
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