Physics Photo of the Week

August 27, 2010

On a Clear Day...

As can be deduced from the position of the motor boat in these two images, they were taken one almost immediately after the other.  This day was an exceptionally clear day (August 7, 2010) at Willoughby Lake in Vermont when there were virtually no haze "particles" - microscopic water droplets, pollutants, etc.  The far shore is about 3 miles distant and the pointed mountain (Burke Mountain) is about 14 miles distant.  Why is the left hand photo much less "hazy" than the right photo?

The clearer photo on the left was taken looking through a polarizing filter oriented for maximum contrast.  The polarizing filter on the right was rotated 90 degrees from the optimum orientation.  This demonstrates that the bluish tint of the distant mountains is polarized.  With only pure air - no particlulates, no aerosols - between the scenery and the camera, sunlight is scattered from the air molecules.  This is known as Rayleigh scattering and the scattered light is highly polarized.

The schematic diagram at right explains the polarized effect of Rayleigh Scattering.  The incident light from the Sun is unpolarized.  That means the vibrating electric field of the light wave vibrates in all directions perpendicular to the direction of propagation.  When the light strikes an air molecule, it causes the electrons in the molecule to vibrate in all those directions.  When an observer views the scattered light in a direction perpendicular to the incident light (as shown), only the vibrations of the molecule perpendicular to the direction of scattering contribute a light ray in that direction.  The vibrations in the same direction as the direction of scattering do not produce any light in that direction.  Think of producing a wave on a rope by shaking one end of the rope.  The rope has to be shaked perpendicular to the axis of the rope in order to produce a wave. 

On a hazy day when there are aerosols and particulates, the light is scattered off the haze particles - much larger than molecules.  Light scattered off large, irregular particles (leaves, snow, dust) is generally non-polarized and much more intense, thus high haze indicates the presence of particles in the atmosphere.  Polarized air light, as in these photos, indicates almost pure, unpolluted air.




Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu. 

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.


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