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Feb 1, 2010

Why snow is white?

The reason we see snow in the first place is due to light. As snow falls through the atmosphere and lands on the ground, light is reflected off the surface of the ice crystals. Since the snow has multiple facets, some of the light is scattered.

Visible light from the sun is made up of a series of wavelengths of light on the electromagnetic spectrum that our eyes interpret as different colors. When light hits an object, different wavelengths of light are absorbed and some are reflected back to our eyes. To complicate matters, light passing through ice will not continue through the ice crystal without first changing directions or reflecting off an interior angle within the ice crystal.

No one really ever sees one snowflake at a time. Most of the time, we see huge collections of millions of snowflakes on the ground. As light hits the snow on the ground, there are so many locations for light to be reflected, that no single wavelength of light gets absorbed or reflected with any consistency. Most all of the white light from the sun hitting the snow will reflect back and still be white light. Therefore, snow on the ground appears white.

One other important point to remember is that snow is indeed tiny ice crystals. Ice itself is not transparent like the glass in a window, but translucent. Light does not pass through ice easily. Instead, it bounces around back and forth within the ice crystals. As the light inside an ice crystal bounces around off the interior surfaces, some light is reflected and other light is absorbed. With the millions of ice crystals in a layer of snow, all this bouncing, reflecting, and absorbing leads to a neutral ground. That means there is no preference to one side of the visible spectrum (red) or the other side (violet) to be absorbed or reflected. The sum total of all that bouncing leads to white.


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