Photo of the Week
is a slab of ice from water in the dog dish that had frozen overnight
back in December, 2005. My son Michael is holding it up against
the clear blue sky, and polarizing sunglasses are held in front of the
camera. You will get the same effect if you wear polarizing
sunglasses while holding the slab of ice up to the blue sky.
The secret of the vivid crystallite
effects lie in properties of polarized light and the properties of blue
sky. The blue light in the sky is polarized. The blue color
arises from Rayleigh scattering of sunlight from fluctuations in
density of the earth's atmosphere. In the photos above, the sun
is low to the horizon in the left side of the photos and the camera
looks straight up toward the zenith through the slab of ice. The
scattered sunlight is polarized in the "y-direction" of the
photos. In the left-hand photo the polarizing lens is set to
allow only light polarized in the "x-direction"
(left-right). Because the light wave vibrations from the
sky are in the y-direction, and the polarizing filter allows only the
"x-direction" of vibrations to go through, the sky appears dark.
The ice slab with its myriad of crystallites is essentially positioned
between two polarizing filters: the sky and the sun-glasses. For
the filter on the right, the polarizing filter, or "lens", is oriented
in the y-direction, the same direction as the sky polarizion, thus
allowing for a bright sky.
These images indicate that the ice slab is crystalline in its
make-up. The ice crystals are called "bi-refringent" i.
refractive index, or speed of light, is different for the different
orientations of polarization. Light polarized parallel to a main
axis of an ice crystal has a different speed in the medium from light
polarized perpendicular to the main axis. As a result the
different components will emerge and
add together as if the plane of
polarization has been rotated. Thus, if the polarizating filters
are "crossed" in the left hand photo, some of the crystals will rotate
the plane of polarization so that the emerging light is parallel to the
next filter, thus transmitting light. The amound of rotation
depends on many parameters: the orientation of the crystal to the
polarized light, the thickness of the crystal, and the color of the
light. Notice in the right-hand photo, the light-dark
crystallites are reversed. This
technique is often used to
observe strains in deformable materials.
right is a picture of of the ice slab
held up to the sky with no polarizing
filter. Notice how clear the ice is,
except for some sticks and leaves that had become frozen in it.
Next time you are wearing your
polarizing sunglasses, take a look at
the blue sky and rotate the lenses. You'll notice
of the blue sky in the area about 90 deg from the sun.
Photographers often make use of this technique with polarizing filters
to enhance the contrast between the blue sky and clouds. The
light scattered from the clouds is non-polarized.
At left is the dog dish with the frozen
slab of ice still in it.
Notice you can see some crystallites in the ice
reflected sky light in the dog dish.
All photos by Donald F. Collins
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.
Please send any photos to firstname.lastname@example.org.
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the Week Archive.
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