Physics Photo of the Week

May 9, 2012

Glacial Lake Color
Lake Louise, Banff National Park, Canada is fed by melt water from the Victoria Glacier that is visible in the crotch of the vee in the center of the photo.  Click on the image for a larger photo. 

The physics demonstrated by this picture is the turquoise blue of the lake water characteristic of all glacier-fed lakes and rivers.  Fine rock flour - caused by the relentless grinding of the rocks in the glacier - is suspended in the meltwater and is chiefly responsible for the spectacular blue-green color.  Detailed physical explanations are rather lacking, however.  Guidebooks state that the color is the result of diffraction of light by the fine particles.  However, the size of the particles have been measured, and the smallest particle sizes are about 5 microns across - about 10 times the wavelength of visible light.  Therefore diffraction of light is not a principle contribution. 

A simpler explanation than diffraction is reflection of transmitted light by the suspended particles combined by the reflectivity of the rock composition that makes up the particles.  Imagine a swimming pool (even an indoor pool) containing clear water with white tiles lining the pool.  The water reflected from the white bottom appears blue due to weak absorption of light by water in the red region of the spectrum.  (See "Blue Ice" PPOW for March 12, 2010).  The fine rock dust suspended in glacier-fed lakes acts the same as the bottom of a pool, except the reflective surfaces are at many different depths in the water - not just on the bottom.  Deeper particles reflect light that is bluer than the shallow particles, thus the cumulative effect of light reflected from all depths is a combination of different ratios of all colors relative to blue light.  The composition of the rock flour is another major contributor to the water color.


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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