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 incid
ent
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.
Click
here
to
see the Physics Photo
of
the Week Archive.
Observers are invited to submit
digital photos to:
