Physics Photo of the Week

September 12, 2008

Polarized Air Light


Notice how the mountains in Kootenay National Park, British Columbia, Canada look clearer and more "contrasty" in the left-hand image than in the right-hand image.  The right-hand image looks more hazy, although the air was very clear and hardly any haze was present.  The mountains in the center are about 10-15 km distant, plenty of space for intervening air to scatter sunlight.  The scattered sunlight from a pure pollution-free atmosphere is blue - hence the blue sky.  The air volume between the camera and the distant mountains appears blue due to the same reason the sky is blue.  This effect is often referred to as "Rayleigh Scattering" first explained by John William Strutt (Lord Rayleigh).  All distant mountains in pure air appear blue due to Rayleigh Scattering.  Sunlight shining on the air between the camera and the mountains is scattered towards the camera.

The effect was made more visible by means of shooting both photographs through a simple polarizing filter.  The scattered light (air light, blue sky) is polarized.  This means that the light waves are vibrating only in one direction.  Looking through a polarizing filter that is oriented to block the polarized air light, the distant mountains show much better.  Such is the case for the photo on the left.  The polarized airlight is blocked, making the mountains appear much darker.  On the right-hand image, however, the polarizing filter is oriented so that the air light is not blocked (the filter axis is parallel to the plane of the light wave vibration).  Because so much scattered air light (predominantly blue) enters the camera, the blue light overwhelms the image and causes the distant mountains to be less visible.  Notice also that the spruce trees in the foreground are much less visible in the right-hand photo.  The scattered air light makes the scene so bright that the automatic exposure sets the shutter speed to a much smaller value.  Thus the foreground objects are darker than they should be because there is no air light in front of the foreground objects.


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.

A polarizing filter oriented to block the polarized scattered air light blocks the air light and allows more of the background mountains to show.  Photographers often use polarizing filters to increase the contrast between the blue sky (polarized) and clouds and distant mountains (un-polarized).  However, close attention must be given to the axis of the polarizing filter. 



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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