Built on Facts

A few days ago we had an interesting discussion about the actual nature of light waves with respect to the informal qualitative presentation of light waves found in intro textbooks. Because I have the best set of physics blog readers in the world, a fascinating discussion ensued with CC and Neil among others. One of the points that was brought up was polarization, by more than one commenter:

How do polarized lenses work then? I was always under the impression that they worked on the fact that light waves had a physical ‘up and down’ movement.

And I’m not at all surprised that this impression is common. All over the place from textbooks to lectures to websites you see example pictures that look like this:

[Image removed per request]

What’s good about the picture is that it conveys the idea of what it means for a polarizer to select a polarization. What’s bad about it is that the “picket fence” metaphor doesn’t really have anything to do with the actual physics of light polarization. Instead it furthers the exact misconception we talked about in my earlier post – that the crests and troughs of the wave have a physical up-and-down extent in space. They do not. The ups and downs represent the strength and direction of the electric field at that point, that’s all.

Take a piece of polished aluminum and shine a flashlight at it. The light reflects back at you. The underlying physical reason is that the oscillations of the electric field of the wave at the surface of the metal makes the conduction electrons in the metal slosh back and forth in response. And accelerating charge itself generates an EM wave. Do the math, and you’ll find out that it works in such a way as (ideally) to generate an outgoing version of the incoming light wave – a reflection.

Now imagine that the grid of slits in the picture above is made of metal. The electrons can slosh back and forth along the “wires” making up the grid, but they are blocked from freely oscillating if the incoming wave happens to be pushing the electrons perpendicular to the wires. Thus reflection will only happen for light polarized with the wires, and the light transmitted will therefore be polarized perpendicular to the wires. Which is the exact opposite of the picket fence model in the picture.

In total the rope and fence analogy is quite misleading, and though it’s a nice picture I’m pretty sure it would be better to scrap the whole thing.

Comments

  1. #1 Eric
    May 29, 2009

    It’s especially confusing when you throw quantum effects in. It’s a great demo in a high school physics class to put two filters perpendicular and show that no light gets through… until somebody puts a third one in between the two at a 45 degree angle, and somehow the light does get through.

  2. #2 Greg Laden
    May 29, 2009

    The ups and downs represent the strength and direction of the electric field at that point, that’s all.

    Direction? Like, up and down?

  3. #3 Greg
    May 29, 2009

    If I understand correctly, it’s direction like pointing up and down, rather than like moving up and down.

  4. #4 Uncle Al
    May 29, 2009

    1) Bounce some light off an aluminum sheet at a shallow angle and analyze with a rotating linear polarizer.

    2) Bounce some light off a glass pane at a shallow angle and analyze with a rotating linear polarizer.

    3) Bounce some light off a second surface mirror at a shallow angle and analyze with a rotating linear polarizer.

    Did you expect some clever contrast of results?

  5. #5 dreikin
    May 29, 2009

    It’s especially confusing when you throw quantum effects in. It’s a great demo in a high school physics class to put two filters perpendicular and show that no light gets through… until somebody puts a third one in between the two at a 45 degree angle, and somehow the light does get through.

    I wouldn’t really think that’s a quantum effect (as described, anyway) – you can do the same thing via mechanical model. Set a ball attached on a rod so it can only slide along the rod. Set the rod at a 45 degree angle to the ground. Now put a board parallel to the ground, and with a linear hole in the middle long enough that the rod can pass through it for a wave motion. Move the board up and down, and the ball will still move along the rod, changing some of the energy into horizontal motion.

    As for the diagram – that also obscures that the wave can also have various degrees of longitudinal orientation with respect to the direction of propagation, as well as, or even without, the transverse orientation.

    Uncle Al: For those of us to lazy to do so, and/or without the equipment to do it anyway, what’re the results?

  6. #6 Wayne
    May 31, 2009

    Posts like this are the reason I keep coming back here. I was always unsatisfied with the picket-fence explanation. I suppose I should have looked for a better explanation myself, but life intervenes. Today in just a few minutes I finally got it and learned the mechanism of reflection too. Thanks for doing this for us all.

  7. #7 Paul Murray
    May 31, 2009

    Analysinw what’s *wrong* with the picket-post analogy is perhaps a nice way to introduce QM.

    After all – if the pickets blocked all but thise waves going exactly up-and-down, then almost no light would get through a single polariser.

    But if it was entiely probabilistic – a vertical picket has a probability p of letting a wave through that’s at 45 degrees, then crossed polarisers would not block all the light.

    You need QM to explain it.

  8. #8 confused
    June 1, 2009

    Eric: where could I look for a good explanation of this ’3 polarizer’ experiment?
    I’ve always had a feeling it could be explained with linearly polarized light being a superposition of left and right circularly polarized light – but now I’m confused again…

  9. #9 dreikin
    June 1, 2009

    You need QM to explain it.

    Or an understanding of it like I stated above, where no such probabilities are needed. Of course, I’m basing mine off the limited example of this post, so more info could show that untenable..

  10. #10 Crux Australis
    June 7, 2009

    Great explanation! How about plastic polarizing lenses; no freely moving electrons.

  11. #11 uber
    June 18, 2009

    thanks for the great explanation (and for quoting my question from your last post).

    We talked about it here and thought it might have more to do with what one would see when viewing a laptop screen at an angle. Polarized lenses only let light through if it hits at a specific angle….i guess we were wrong…somewhat?

    thank again and keep the knowledge flowing!

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    March 3, 2010

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  14. #14 Han Hyo Joo
    April 16, 2012

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