Physics Photo of the Week
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.
Photo of the
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.
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