Ice and Medicine at the end of the earth

Why snow is white but glacier ice is blue

Thanks for all who responded. There were many great explanations!

Below, is mine, but first, I wanted to announce the winners:

General entry: Dana Roberts

AFSA Block 1: Tim

AFSA Block 2: Jacob

AFSA Block 3: Levi

AFSA Block 5: Rachel

Humor Award: Melanie Parker Berg – “Snow is white because that was the color of Snow White’s face, and ice is blue because blue raspberry is the preferred shaved ice flavor of Antarctic penguins.”

Winners will receive Antarctic Dollars and USAP Patches.  Thanks again for participating!

Q. Why is snow white but glacier ice blue?
A. This question is more complicated than it may first seem.  Snow and glacier ice are both made of ice and air, just in different amounts.  To understand why we see different colors, we’ll first discuss two main concepts.

1. Absorption of Light by Ice – White light is an even mixture of all the wavelengths of visible light (from blue to red).  Light is absorbed as it travels through the ice.  Ice absorbs relatively little visible light (either blue or red), but it still absorbs red light ten times more than blue light.

2. Scattering light back to the surface – Snow is composed of a bunch of ice grains with air in between them.  In glacier ice, those air pockets have been compressed into small bubbles.  When light enters snow or ice, it is scattered whenever it reaches an ice/air interface.  So in snow, there is a lot of air and a lot of opportunity for the light to be scattered.  But in glacier ice, there is less air and fewer opportunities for light to scatter.

Putting these two concepts together: Snow is white because the light that enters the snow is quickly scattered back out.  The light travels through so little ice that almost no light (red or blue) is absorbed.

Glacier Ice is blue because the light that enters the glacier ice travels much farther in the ice than it did in the snow.  This gives the ice time to absorb more red than blue light.  So when the light returns to the surface, it is lacking red light, making it appear blue.

This is a photo from an email of iceberg photos that Deb sent to me.  From how blue the ice is we can get a good estimate of how much air is trapped in the ice.

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5 responses

  1. dana

    Woohoo!! I won!

    December 21, 2010 at 3:38 AM

    • It was a tough one. Nice work!

      December 21, 2010 at 10:41 AM

  2. Melanie

    sweet! I can’t wait to see the Antarctic dollar!

    December 21, 2010 at 5:54 PM

  3. mom

    Congratulations to the winners!!

    December 22, 2010 at 5:41 PM

  4. PRIZES ARE IN THE MAIL TODAY!! Waiting for the next flight off the ice. Congratulations to the winners and participants!

    December 27, 2010 at 11:05 AM

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