Physics Photo of the Week

February 14, 2014

Why is Snow White?
The composition of snow is pure water in the solid phase - ice.  Ice is clear like water.  Why, then, is snow white and not colorless like water?

The answer comes from the fact that snow consists of millions of tiny crystals of ice - snowflakes.  Each snowflake has many points and surfaces.  So a snow layer such as this scene consists of millions and millions of tiny crystal faces of colorless ice.  Each surface of a clear crystal reflects part of the incident light.  This is called "specular reflection".  Specular reflection can be seen on a piece of clear ice in the photo below.  On the left end of the piece of ice, the ice is clear, but a distant light is partially reflected by the surface.  The same partial reflection enables us to use a clear window as a partial reflecting mirror.  The amount of partial reflection depends on the index of refraction of the transparent material.  The interior of the ice piece is all "milky" due to the myriads of tiny air bubbles in the ice.  The many air bubbles are tiny surfaces in the ice that partially reflect the light in many directions - giving it a "milky" appearance.  In the snow, the specular reflection from the millions and millions of tiny surfaces within the snow eventually reflects virtually all the incident in random directions  Thus, even though the individual snow crystals are colorless, the scattering of all the incident light in all directions gives the snow the white color - the same color as the source of the light impinging on it.

This photo commemorates the first major snowstorm in Western North Carolina for several years. 

Don't forget to get a Valentine for your sweetheart!

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

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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