Physics Photo of the Week

February 15, 2013

Ordinary Ice
This is a photo of a sheet of ordinary ice that had formed in a rectangular bucket overnight in mid-January, 2013.  This piece of ice was very clear when held up to normal light.

However, if you wear ordinary polarizing sunglasses, hold the slab of ice up to the blue sky on a clear day, the ice exhibits these amazing crystalline patterns of contrasting colors!

The common blue sky is polarized.  It consists of sunlight scattered by the pure air molecules in the atmosphere.  (See PPOW May 8, 2009 for a demonstration of the polarized blue sky).  The light from the polarized blue sky is generally vibrating horizontally if the Sun is high in the sky.  That is, as you are looking toward the blue sky in this picture the light waves that reach your eyes are vibrating left-to-right.  Your polarizing sunglasses are such that they block the light that is polarized horizontally, but transmit the vertically polarized light.   Thus the blue sky in this photo appears quite dark (May 8, 2009 PPOW).  The camera is wearing a polarizing filter similar to polarizing sunglasses an perpendicular to the horizontal plane of blue sky polarization.  The transparent ice is essentially placed between "crossed polarizers" - a common technique for examining crystal structures in microscopy, thermal stresses in plastics and glass, and even the stresses in clear adhesive tape.  

The image at the left shows a slab of ice formed on the night of Feb. 13, 2013 held up to the sky with no polarizing filter.  The ice is transparent and hardly visible without the polarizing optics!

Many clear crystalline compounds (water ice included) are bifringent.  They exhibit different refractive indices for different directions for the vibrating waves of light.  This is primarily due to the different spacings between molecules in different directions.  Basically if the molecules are closer together in one direction, the speed of light for waves polarized in the direction in which the molecules are closer together is slower than for the light waves polarized parallel to the less-dense direction.  If the principle axis of the crystal is oriented parallel to one of the polarizer axes, the light is reduced.  If the principle axis is at an angle about half-way between that of the crossed polarizers, the light is often colored due to interference of the emerging light.  The animation  here alternates between two perpendicular orientations of the ice sheet.  Notice how the large distinct crystals in the center of the slab alternate between light and dark, or blue or red.


Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature 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.

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

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