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Thread: Polarizing filters

  1. #1 Polarizing filters 
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    http://en.wikipedia.org/wiki/Polarizing_filter

    I just read over this and am a little confused. Polarization is the orientation of the electric and magnetic component of a photon relative to a surface, right? So...why would things like parts of the sky polarize light, and how? It seems strange that some things align all of the light and others do not.

    As for the polarizing filters...what is their degree of freedom? Do they let all light in besides a certain range, or filter all light out besides a certain range? How many degrees wide is this range? Looking through a single filter, you seem to receive most of the light as depicted in the wikipedia photographs; yet, in my high school physics class, the teacher demonstrated 2 polarizing filters 90 degrees out of alignment completely blocking the light. Do polarizing filters actually alter the polarization of the incoming light, blocking out only the portion that is too much out of alignment to be altered? So, when you align the light along an axis using one filter, and put a filter 90 degrees out of alignment with it, are you essentially forcing the initial light into being at the beyond alteration threshold for the second filter?

    Also, can you buy a polarizing filter that retains the same polarization regardless of how it is rotated?

    Thx.


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  3. #2 Re: Polarizing filters 
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    Quote Originally Posted by Cold Fusion
    I just read over this and am a little confused. Polarization is the orientation of the electric and magnetic component of a photon relative to a surface, right? So...why would things like parts of the sky polarize light, and how? It seems strange that some things align all of the light and others do not.
    The air doesn't really polarise light, it diffuses light of the sun in all directions. The light of the sun is not polarised, but the waves are orthogonal to its direction. If it is diffused, it can still only be orthogonal to that original direction, which excludes polarisations of that light that aren't orthogonal.

    At least, that's how I understand it.
    Quote Originally Posted by Cold Fusion
    As for the polarizing filters...what is their degree of freedom? Do they let all light in besides a certain range, or filter all light out besides a certain range? How many degrees wide is this range? Looking through a single filter, you seem to receive most of the light as depicted in the wikipedia photographs; yet, in my high school physics class, the teacher demonstrated 2 polarizing filters 90 degrees out of alignment completely blocking the light. Do polarizing filters actually alter the polarization of the incoming light, blocking out only the portion that is too much out of alignment to be altered? So, when you align the light along an axis using one filter, and put a filter 90 degrees out of alignment with it, are you essentially forcing the initial light into being at the beyond alteration threshold for the second filter?
    A polarisation filter absorbs the component of light perpendicular to its polarisation, and only allows a single angle to pass. E.g if you have polarised light and pass it through a filter with a 45 angle, cos(45) (the projection of the light on the direction of the filter) of the light will pass through.

    This has the interesting effect that two sheets at 90 block all light (cos90=0), but if you place a sheet at 45 between those, it does transmit light again (cos45*cos45).


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  4. #3  
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    Thanks for your response.

    I also came about "Linear" and "Circularly" polarized filters. I'm having a hard time wrapping my mind around this idea despite wiki's explanation. Does the difference lie in that circularly polarized filters essentially take a single linear polarization, and bend it into a more ambiguous circular form so that it does not appear polarized, but has its net vague structure composed of concise photonic orientations?
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

    -Einstein

    http://boinc.berkeley.edu/download.php

    Use your computing strength for science!
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  5. #4  
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    I didn't mention circularly polarised light because it is more complicated, and I am not entirely comfortable with the material.
    As I understand it, circularly polarised light does have a polarisation like linearly polarised light, but the orientation changes as the beam progresses, more specifically, the orientation rotates at a certain rate around the axis of the beam.

    If you pass circularly polarised light through a linear filter, the filtering depends on de position of the filter. At one point all the light might go through, and a bit further, where the beam has rotated 90 nothing passes through.

    I'm guessing recent 3D-movies work with circularly polarised light and circular filters, rotating in opposite directions for each eye.
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  6. #5  
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    ......so, if you were to position the second polarization filter at a different distance from the initial circularly polarizing filter, the amount of light attenuated would change?
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

    -Einstein

    http://boinc.berkeley.edu/download.php

    Use your computing strength for science!
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  7. #6  
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    yes
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