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Intensity of light after passing thru polaroid

  1. Sep 12, 2014 #1
    1. The problem statement, all variables and given/known data
    Two pieces of polaroid are arranged so they are initially parallel . One of the polaroid is rotated until the intensity of the light beam from the two polaroid is 1/3 of its initial intensity . Calculate the angle turned thru the polaroid

    2. Relevant equations

    3. The attempt at a solution
    i assume it's a non-polarised passing thru the first polaroid , then the second polaroid is rotated.
    so i have Intensity after passing thru 1st polaroid (I 1 ) = 0.5 intensity of non-polarised (I 0)...
    for light ray emerging from second polaroid = (I 2 ) = 0.5(I 0) ( (cos θ )^2) .finally i found out my θ = 35.3° the ans form the book is 54.7°

    why i am wrong?
    the initial light ray is polarised or non- polarised? how to determine it? it's not stated in the question....
  2. jcsd
  3. Sep 12, 2014 #2


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    It sounds like they did not phrase the problem correctly. With the precise phrasing you wrote, you are correct (assuming initial unpolarized light). But if interpret the question as meaning: at what angle is the intensity equal to 1/3 it was before entering the second polarizer (in other words, by "initial" we mean before the second polarizer, not before the two polarizers) then we get the answer they give.
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