# Physics Photo of the Week

## February 18, 2006

### Polarizing Ice

This 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. e. the 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.

At 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 the darkening 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 from
the reflected sky light in the dog dish.

All photos by Donald F. Collins

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.