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.