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Polarisation (light on water surface)

  1. Jan 2, 2017 #1
    1. The problem statement, all variables and given/known data
    A student looks at the sunlight reflected off a puddle of water. She puts a polarising (Polaroid) filter in front of her eye. As she rotates the filter the puddle appears darker then lighter.

    Explain this observation

    2. Relevant equations


    3. The attempt at a solution
    Here's my answer:

    "As the student rotates the filter so it is in the same plane of polarisation of the polarised light, the wave will be absorbed (ie blocked) thus the puddle will appear darker.

    And when it is rotated so it is perpendicular to the plane of polarisation of the reflected wave, the wave is no longer absorbed so the glare remains and the puddle will appear brighter''
    ----

    But according to the mark scheme this would be wrong because ''When planes are parallel puddle appears light OR when perpendicular puddle appears dark''

    where have i gone wrong? since the light is reflected horizontally, wouldn't the filter that is perpendicular to the light NOT block out the light, thus making the puddle appear brighter? (because the glare remains)
     
  2. jcsd
  3. Jan 2, 2017 #2

    berkeman

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    Staff: Mentor

    I think you have the concept right, but it is probably just a convention in how the "direction" of the polarizer is defined. I'm most familiar with saying that light that is polarized in the same direction as the polarizer makes it through. But you are correct that the physical stretching/alignment of a Polaroid filter is the direction that absorbs the light. Have a look at this wikipedia page, for example:

    https://en.wikipedia.org/wiki/Polarizer
     
  4. Jan 3, 2017 #3
    polarized_ligth_water_reflection.jpg

    Okay using the above information, is this valid:

    "Since reflected light is horizontally polarised, if the polaroid is oriented so it is perpendicular to the plane of the reflected light, the puddle will appear darker because the reflected light has been absorbed, ie blocked, because only light that is in the same direction as the polaroid is allowed through.

    Whereas when the polaroid is oriented so it is parallel to the reflected light, the glare remains because the reflected light is in the same plane as the polaroid so it is let through thus the puddle appears brighter."
     
  5. Jan 3, 2017 #4

    berkeman

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    Staff: Mentor

    The way I read the wiki article, when the plane of the polarized light is parallel to the stretched direction of the Polaroid film, it is absorbed.
     
  6. Jan 3, 2017 #5

    berkeman

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    Staff: Mentor

     
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