Why Do Polarized Sunglasses Work?

i-8c0e4f5fc95209bb67d3d3a39840d5f6-electric_field.jpgIn the previous post about light polarization, I promised to post an explanation of why it is that “Polarized” is a selling point for sunglasses. Given that sunlight is unpolarized, the only obvious benefit would be that polarized sunglasses will automatically block half of the light hitting them, but it’s actually much better than that. To understand why they work, though, we need to talk about how it is that light waves are produced and propagate in a medium.

Sticking with the classical picture of light as an electromagnetic wave, you can understand the production of electromagnetic waves by just looking at the electric field of a simple charged particle, like an electron. The picture at left shows the basic idea: you have some object, with some charge, and there are field lines raditating out from it in all directions (I’ve only drawn a few, to give you the basic idea).

Now, let’s imagine what happens when you suddenly move that charge some distance. When that happens, you get a picture that looks like this:

i-a21a49f08e074025dde6459484d3d26d-kinked_field.jpgIn this picture, the dotted lines represent the original field lines, and the blue lines are the field from the new position. If you’re close by the charge, everything looks more or less the same– it’s still just lines radiating out from the central point.

But we need to remember our Relativity, and recall that forces propagate at finite speed. If you’re far away from the charge, you don’t know that the charge has moved until the new lines reach you, and that takes a time equal to the distance you are from the charge divided by the speed of light. So, at large distances, you see the green field lines, which look just like the original.

Between the blue lines radiating out from the new position, and the green lines radiating out from the old position, there’s a “kink” in the electric field, shown by the red lines. This represents the transition region, during which the charge was being moved from one position to the other. That “kink” propagates outward at the speed of light, and is the beginning of a light wave. If, rather than making a single short move, you were to wiggle the charge back and forth in a regular manner, you would generate waves, moving out in all directions.

Now, look at the pattern of the “kinks.” The “kink” in the horizontal field line is rather large, but the “kink” is much smaller in the lines that go off at an angle. And if you look at the vertical line, you see that there’s no kink at all– the charge shifted exactly along the field line, so there’s no change. This is a very general result: if you shake a charged particle back and forth, you get light waves radiating out from it in every direction except along the direction of motion.

What’s this got to do with sunglasses? Well, you can think of light propagating through a medium as a wave shaking the charges in the material back and forth– the elecrons are actually bound into atoms, but they still move, and emit light. You can picture the wave that travels through the material as the sum of all those individual waves, an idea known as Huygens’s Principle– the waves emitted by the atoms makeing up the material interfere constructively in the direction of propagation, and destructively everywhere else.

Now, think about what happens when the light hits a boundary. At a boundary between two different media, you actually get two possible paths: a transmitted wave that passes into the new material (bending a bit as it enters, which is called “refraction”), and a reflected wave that bounces off the boundary and goes back out into the original medium. Both of these beams are built up from the interference of the light emitted by individual atoms.

What does this have to do with poalrization? There’s a spiffy animated applet at Davidson that demonstrates. If you shine horizontally polarized light at a surface, it will reflect and transmit no matter what angle you pick. If you use vertically polarized light, though, there is some angle at which the reflected beam from the surfacewould need to go exactly along the direction of oscillation of the wave. Which can’t happen, because the individual atoms don’t emit any light in that direction, so you only get transmitted light, and no reflected light.

That means that for any horizontal surface, there’s an angle at which vertically polarized light will not reflect. This, in turn, means that light reflecting at that angle will be horizontally polarized, even if the light source illuminating the surface is unpolarized. This is called “Brewster’s Angle” after a Scottish physicist who figured out the rule for finding the angle.

If you wear glasses made from vertical polarizers, then, light reflecting off surfaces at Brewster’s angle will be completely blocked, while light that bounced off at some other angle will get through. And light reflecting off at angles close to Brewster’s angle will be partially polarized, and thus most of it will get blocked.

This is why polarized sunglasses are particularly important for fishermen: sunlight reflecting off the surface of water will be polarized, and polarized sunglasses will block most of the glare, allowing you to see below the surface of the water more effectively. They’re also good for driving, as sunlight reflecting off the road some distance ahead will be blocked by the polarizers, reducing the strain on your eyes.

And this is why I’m looking for a new pair of polarized sunglasses that aren’t incredibly ugly– I’m going to be on vacation in the Caribbean next week, and anything that reduces the glare off white sand and blue water is a Good Thing…

Comments

  1. #1 mollishka
    July 24, 2007

    Similarly, if you’re wearing polarized sunglasses, you won’t see rainbows, unless they’re at odd locations or somesuch (since rainbows are mostly polarized light tangential to the bow).

  2. #2 cisko
    July 24, 2007

    I had to give up on polarized sunglasses, because they made it impossible to read the LCDs in my car, on my camera, on my iPod, etc. I suppose I could get a second pair…

  3. #3 Mark P
    July 24, 2007

    I had the same problem as cisko, plus, when I rode a motorcycle with a full-face helmet, I couldn’t wear polarized sunglasses and still use my face shield because of all the weird interference patterns the two created. Unpolarized glasses worked fine. And I have stuck with unpolarized sunglasses ever since.

  4. #4 Matt
    July 24, 2007

    Maui Jim makes some nice looking polarized sunglasses. As does Revo. Both are pretty pricey, but that’s the cost of lookin’ good.

  5. #5 Temple
    July 24, 2007

    Love the Ray-Ban polarized.

  6. #6 Michael Norrish
    July 24, 2007

    I read recently that there was a suggestion that car headlights be put behind polarising glass, and that windscreens be polarised the opposite way so as to reduce oncoming glare effects… This was suggested 50+ years ago (maybe even by Polaroid man), but still hasn’t been done.

  7. #7 Pseudonym
    July 24, 2007

    This, incidentally, is one of the reasons why fancy digital 3D cinema projectors use circularly polarised light. If they used linear polarisation, then the brightness of the image would differ in each eye depending on where in the cinema you sit. (It still does, but the effect isn’t as pronounced.)

  8. #8 cisko
    July 25, 2007

    And polarizing filters for autofocus cameras have to use circular polarizers too, or the autofocus will decide to stop working.

  9. #9 Carl Brannen
    July 25, 2007

    Psuedonym wrote: “This, incidentally, is one of the reasons why fancy digital 3D cinema projectors use circularly polarised light.”

    Not always so. I saw the dinosaur movie at the IMAX theater at the Pacific Science Center last Saturday. It was 3D and required 3D glasses. I borrowed a pair from my friend and showed her that they were linearly polarized. I’m too old to tilt my head 90 degrees to swap the left and right images (and risk barfing on my friend), but I did tilt 45 degrees to verify that the images split.

  10. #10 Clay B
    July 25, 2007

    Thanks for the explanation. I’ve seen the derivation for why the H and V reflectivities differ from solving the boundary conditions for the E and B fields, but never made the physical connection until now. Incidentally (har,har), this is important in remote sensing of the Earth and atmosphere using microwaves.

  11. #11 Anonymous
    July 25, 2007

    This article is a good example of the principle that if you try dumbimg down physics, you reach the point of diminishing returns almost instantaneously.

  12. #12 DBH
    July 26, 2007

    Re: headlights and polarized windshields, remember that a polarized windshield will filter 50% of all light that didn’t come from headlamps…thats an issue. Also, the light from YOUR headlamps, backreflecting off a flat metal surface or corner cube will be invisible!

  13. #13 JPinNH
    July 26, 2007

    A polarized light question for the physics mavens: I’ve long noticed, but never seen described, the following phenomenon:

    When wearing vertically polarized sunglasses, facing more-or-less North, and looking at a clear blue sky, tilting my head just a few degrees causes the appearance of the sky to darken noticeably. It doesn’t happen when looking in any other direction.

    What’s going on here?

    Cheers,

    JP

  14. #14 Dan L
    July 26, 2007

    JPinNH, blue sky light, which is rayleigh scattered, is
    polarized, relative to the sun’s position. The closer your
    viewing angle is to perpindicular to a ray from the sun to
    you the more polarized it is.
    Polarized sky light also explains the problem that commenter
    number 3, Mark P, had with his motorcycle visor and polarized
    lenses. The plane of polarized skylight is differentially
    rotated by strained regions in the visor, and hence the
    strained regions show up as blotches if you are wearing
    polarized glasses.

  15. #15 JPinNH
    July 26, 2007

    Dan L,

    Thanks very much! I’ve been wondering about that for years, and I’m too embarrassed to describe my internal attempts to explain it in terms of the earth’s magnetic field…

    Cheers,

    JP

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    June 1, 2008

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  17. #17 Charles
    January 22, 2009

    This may or may not make sense, but I haven’t gotten a straight answer from an opthamologist or an optician, other than it can’t be done. Since cataract surgery a few years ago I have night time glare problems while driving, especially during rain, and with the newer superbright headlamps mounted high in the field of vision, on SUVs. Why would non-tinted, polarized lenses on glasses not solve part of this problem. I have already tried the anti-glare coatings and find them to be a waste of time and money.

  18. #18 Josh
    February 26, 2009

    Hi everyone, dont know how I landed here but ive got a question. My brother told me hes heard of this before. Is there a clear film that can only been seen while wearing polarized lenses?

  19. #19 Peter
    March 28, 2009

    Thanks very much! I’ve been wondering about this for years