Physics Photo of the Week

April 21, 2006
Polarized Sky

Something as ordinary as blue sky on clear days with scattered clouds consists of polarized light. Compare the two photographs on the right.  The polarizer is rotated 90 degrees between the two photos.  Notice that the sky is much darker in the top photo than the bottom.  The light from the blue sky is polarized  in the vertical direction.  When photographed through a polarizing filter oriented horizontally, the vertical polarized light from the blue sky is mostly block and appears dark. 

In the lower photo the polarizing filter was rotated so that it allows vertically polarized light to be transmitted.  Notice that the sky is much brighter and the clouds are about the same brightness. 

Photographers often use polarizing filters to enhance the contrast between clouds and the sky - especially for black and white photography.  The polarizing filter technique has a different effect on color photography, however.  The color of the sky adds a color contrast between the clouds and sky without the need for darkening the sky.  The resulting color in the horizontal polarizer image is rather artificial, somewhat more purple.  However, no filter will eliminate the distracting power line from the middle of the photos....

Photos by Donald Collins
The reason the blue sky is polarized is caused by the scattering of light from the sun.  The sun shining on the air molecoles of the sky is non-polarized.  The electric fields of the light waves from the sun vibrate at all angles perpendicular to the direction of propagation.  In the sky photograph, these directions are vertical relative to the observer and in-line with the observer.  Remember, the sun is oriented about 90 deg from the direction the photographs are looking.

The electrons in the air molecules are vibrating in the direction that the waves from the sun are vibrating: vertical, and forward and back relative to the line of sight.  The vertical vibrations can re-radiate (scatter) the light into the direction of the observer.  The vibrations that are in the line of sight don't re-radiate any radiation in the direction that they vibrate.  Consider an object in the distance that is vibrating.  If the object vibrates perpendicular to the the line of sight, we can see it move back and forth.  However, if the object is vibrating forward and back (the same direction of the line of sight), we can hardly see it move.  A similar thing happens with the light, so the vibrations of the air molecules in the line of sight are "invisible".  Only the molecules that vibrate perpendicular to the line of sight are visible - hence the blue sky in the photographs above is polarized in the vertical direction.  A horizontal polarizing filter blocks the vertical polarized light.

Next time you wear polarizing sunglasses, look at the blue sky and cock your head.  You should notice that the sky brightness changes with the angle of your head!  Insects are believed to have polarizing receptors and can detect the polarization of the sky to aid their navigation.
Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an 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.


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