polarization of light

Discussion in 'Photography Beginners' Forum' started by yeti, Jan 8, 2008.

  1. yeti

    yeti TPF Noob!

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    Hi,

    This is probably going to end up being the Weird Question of the Day. I got a circular polarization filter for my camera and I am now playing with it, mostly trying to figure out how it works. I am not sure how many of you approach photography the way I do, probably very few, but I will ask anyway.

    I know what a polarization filter does. I know I can point it at the window at a shallow angle, rotate the filter and suddently WOOSH glare is gone and I can see through the window. This is all great.

    The question is WHY.

    Ok, polarization of light is the function describing the amplitude of the incident light wave. It is always at 90 degrees to the direction of propagation. Sunlight is clearly NOT linearly polarized or my polarization filter would be able to filter it all (it would become all black at 90 degree angle with respect to its polarization). If it is polarized in some other way, then I can write this down as two linearly polarized waves at 90 degrees of each other and out of phase by some angle, which may or may not be constant in time. For the sake of simplicity let's assume it's circularly polarized. For circularly polarized wave the phase is constant 90 degrees.

    Incident light falls upon glass. Narurally some of it is reflected, some is refracted, depending on the refractive index of the glass. I have seen the math behind it and I simply CAN'T remember EVER seeing refraction or reflection ever depending on the polarization function of the wave!!! According to my calculation both circularly and eliptically polarized light always reflects rotating the other way. Both of my linearly-polarized wave functions are reflected equally. Needless to say that means that no polarization filter has any chance of filtering out reflections.

    BUT IT DOES, so I must be wrong.

    I Googled the answer and I found that horizontally polarized waves reflect better at shallow angles. I never found out why, though. So now I have polarization of reflected light dependending on angle of incidence and I don't remember ever seeing this stuff when playing with Maxwell's equations.

    Any ideas? What am I missing?
     
  2. Sideburns

    Sideburns TPF Noob!

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    I'm not even sure that's a question.

    What?!
     
  3. jstuedle

    jstuedle No longer a newbie, moving up!

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    I understand your question, but don't know the answer. As a retired engineer, I too look at think from a different perspective than most photographers. But to be honest this is a subject that I just never pondered.
     
  4. dbrandon

    dbrandon TPF Noob!

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    Very good question, to which i don't know the answer lol.

    I have never seen the effect you speak of (with the reflection in the window), but i understand in fairly simple terms how polarisation works.

    Stab in the dark; you don't think that because the light from the reflection is sourced locally, and then reflected off the glass soon before hitting the filter it has more uniformity about it, meaning the polarisation effect is enhanced. In contrast the light from outside has come from the sun, bounced off loaaadds of objects, and final found its way to your lens, so the waves are going to be at all sorts of random angles, meaning the polarising effect is less ?

    Meh, i don't know ! I have a physics exam on Thursday though, so wish me luck haha ! ;)
     
  5. dbrandon

    dbrandon TPF Noob!

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    If Helen B reads this she will probably be able to answer your question. She seems extremely good at physicsy things !
     
  6. Alex_B

    Alex_B No longer a newbie, moving up!

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    no time to really read this ... but i imagine that the underlying idea is that you see only one coherent wave? sunlight is not one wave, not at all coherent like a laser. so it has no well defined polarisation but a distribution of polarisations. therefore you will always filter out some of those "waves" (wavelets? wave packages?) totally some partially, some not at all. so always some light remains whatever you set your polariser to.


    maybe this is rubbish, even though i am in my own sience, but as said, extremely short of time and full of wine :p
     
  7. Alex_B

    Alex_B No longer a newbie, moving up!

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    oh, and if you do proper electromagnetic waves, your calculations should give you linearily polarised light for the reflected light and for the transmitted light (perpendicular o each other) at the interface between two media of different dielectric constant.
     
  8. Helen B

    Helen B TPF Noob!

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    For reflection at a nonmetallic surface the polarisation of the light does affect the coefficient of reflection, as described by Fresnel's equations. The coefficient of reflection is zero for parallel polarized light when the sum of the angle of incidence and angle of transmission is 90 degrees. For glass with a refractive index of 1.5 the angle of incidence is about 56 degrees for a zero coefficient of reflection for parallel waves. That means that none of the reflected light at that angle will be parallel polarized.

    Hecht's Optics is probably the best standard source for this sort of information, rather than a photographic optics textbook.

    Does that answer your question?

    Best,
    Helen
     
  9. yeti

    yeti TPF Noob!

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    Hi all,

    Thanks for the quick reply. I am sorry for flooding this forum with math lingo.

    Helen B, thanks a lot for your help! I don't think I have seen Fresnel's Equations, but I believe I know enough to understand when reading up on them. I will be revisiting my textbooks. (You can't believe how much I miss doing this stuff now that I no longer have to do it. I never thought I would live to see such a day)

    dbrandon, I don't quite understand what you mean. The light coming to my lens has been reflected and refracted repeatedly and by the time it reaches my apartment it has achieved some unknown (read random) polarization. I wasn't really making any distinction between light from the Sun and light from a lamp in the way it is polarized. All I was doing is wonder what causes the glass to pick one particular polarization to be reflected (so that my expensive filter might work!) :) My math wasn't really taking into account the mobility of the electrons within a dielectric material in the different directions, which is what I believe Fresnel's laws ultimately boil down to.

    Alex_B, coherent light is a light of one frequency. You refer to the waving part of the wave, while I was really talking about its polarization - the part that says which way it is waving. :)

    Once again thanks to all! That's for your help!
     
  10. Alex_B

    Alex_B No longer a newbie, moving up!

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    it is true that coherent light is of one frequency, but also of one polarisation.

    incoherent means you have independent waves. and the result of this is independent frequencies and independent polarisation. So this exactly wa sthe explanation of the sun-light polarisation.
     
  11. dbrandon

    dbrandon TPF Noob!

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    I would ignor my response lol. My knowlegde isn't anything on that of Alex or Helen, so my idea (complete guess) probably makes no real sense at all :lmao::confused:
     

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