Physics Photo of the Week
May
8, 2009
Polarized Sky - Photos by Moira Bullard
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Polarizing
filter
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No filter
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Notice the vivid contrast between the blue sky and the clouds in the
left-hand photo and the rather pale sky in the right-hand photo.
Moira Bullard photographed this scene on a bright sunny day to compare
the appearance of the blue sky both with and without a polarizing
filter. She took these pictures in the early afternoon on April 20,
2009 when the Sun was high in the sky diagonally above the upper right
hand corner in the photos. The camera is looking toward the
southeast in a direction about 90 degrees from the direction to the
Sun.
The blue sky is formed by sunlight scattering off the air 
molecules.
In the direction that is 90 degrees from the Sun direction, the
scattered sunlight is highly polarized due to the vibration of the air
molecules that scatter the sunlight. The drawing at right shows
why the sky is polarized. The blue ellipse represents an air
molecule. The arrows on the ray from the sun indicate the
electric field vibrations of the rays of light coming from the
Sun. There are also electric field vibrations from the sun
directed toward and away from the observer, so we cannot see the
arrows. The air molecule vibrates along the direction of the
electric fields of the Sun's light. This means that the molecule
vibrates both in the direction of the arrows and in line with with the
observer. In turn the vibrating molecule scatters the
sunlight. However, the observer for this drawing only sees the
vibrations in the direction of the arrows, not the forward and back
vibrations. Hence the light scattered from the air molecule is
polarized in the direction of the arrows. So if the polarizing
filter on the camera is oriented perpendicular to the arrows, the light
scattered from the air is blocked. This is the method
photographers use to make blue skies darker in landscape
photographs. The light scattered from clouds in non-polarized, so
the clouds appear just as bright in the polarizing photo making for
vivid contrast.
Moira states that she enjoyed the assignment for Earth, Light, and
Sky. The knowledge she learned about the polarizion effects of
the blue sky will help her become a better photographer.
Physics Photo of the Week will take summer break beginning next week
Friday, May 15 and resume again on Friday, August 21, 2009. Watch
for more pictures created by students that illustrate physics in the
worlds around us.
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:
