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Unknown Light Source Through Polarizer

  1. Mar 27, 2012 #1
    I recently had a homework problem that asked the following. Can someone please explain to me how in the world this can be true?! It makes no sense to me that you can get an intensity that is equal to when light is passed through a vertical transmission axis and a horizontal transmission axis....

    When light from an unknown source, with intensity 1.60 × 10−2 W/m2, passes through a polarizer with a horizontal transmission axis, the intensity of the light transmitted is 8.00 × 10−3 W/m2. When the light from the unknown source passes through a polarizer with a vertical transmission axis, the intensity of the light transmitted is again 8.00 × 10−3 W/m2.

    These are the choices. To me I would think that maybe it would be unpolarized since the electromagnetic fields are randomly oriented so maybe it could go through both, but that does not seem logical to me....

    the light from the unknown source is unpolarized
    the light from the unknown source is linearly polarized
    the light could be unpolarized or linearly polarized

    All I ask is for some clarification because I am finding this section to be very troublesome....
  2. jcsd
  3. Mar 28, 2012 #2


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    If the source were linearly polarised and it just happened to be polarised at 45°, you would get half the power through the polariser in both orientations (but it would need to be 45°. If unpolarised, you would get half power for all orientations. Any of the three choices are reasonable. A2 or A3 could be the answer they're after but the more likely answer would be A1 because it doesn't require a particular pair of orientations. It's a good question because you have to think about it and you would really need to argue a case fro whichever answer you went for.
    In real life, you'd wiggle the polariser about a bit and see if the power varied - if not then the answer is Unpolarised.
  4. Apr 5, 2012 #3

    Claude Bile

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    From the answers you give, the only possible answer is that the light must be unpolarised. There can be no degree of linear polarisation.

    Technically, the light could also be circularly polarised; but since this is not offered as an option, the first answer must be true.

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