Physics Photo of the Week

May 8, 2009

Polarized Sky - Photos by Moira Bullard


Polarizing filter
No filter

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.


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