Expert: James Gort - 5/13/2005

Question
I and some acquaintences have recently noticed that around sunrise and sunset sunlight seems to be more verically polarized than normal.  This causes a more intense vertical light shaft from the sun than normal.  The sun also seems redder, which would indicate some kind of particles causing it.  What do you think?

Answer
Hello Richard,

Good observation!  I'm not sure what's causing it - it could be a volcano, additional air pollution caused by a weather inversion pattern, higher winds than usual kicking up additional dust, or some other cause.  But the reddening and the polarization are definitely related.

The reddened sunset is caused by small particles and clumps of molecules scattering the sunlight.  This process, called Rayleigh scattering, is wavelength-dependent, so the blue light is scattered more than the red light.  At sunset, the light passes through a lot of these small particles, so the blue light is largely scattered out (spread over the entire sky), while the light reaching us on a more direct path is white light minus the blue (or redder).  So the more particles, the redder it appears.

But why is it also polarized?  If the particles (which are not generally spherical, but usually oblong) would be randomly distributed (all orientations equally likely), then the light wouldn't be polarized.  But these particles are subject to earth's gravity, and so are more likely to be oriented vertically than horizontally (they present less air resistance as they fall under gravity).  Then, the light which is "vibrating transversely" or along the axis of the particle "sees" a larger particle, and so is more likely to be scattered - think of the particle as being a larger target for the photon.  This results in plane polarization!

Hope that helps explain it.  Now if we could only figure out what's causing it!

Prof. James Gort