Re: Glacier Color

[Gary Karshner <Karshner@STMARYTX.EDU>]

Wes, Daniel, et.al.
        According to Hecht in his Optics text, almost any time you see blue in the
animal world (blue eyes, blue jay's feathers, even a baboon's blue
buttocks) it is due to Rayleigh scattering. I would assume that the color
of water and glacier ice is the same. The more preferential the scattering
the bluer the color.
        As pointed out larger particles (water drops, large dust) scatters
independently of the wavelength. I remember years ago becoming aware that
the light on the ground had taken on an orangish cast. It turned out the
"clouds" over head were really smoke from distant forest fires and the
sun's disk's light had been scattered so much that it was very orange in
color. I realized that this told me that the "clouds" particles were
smaller that the usual water drops. I saw the reverse happen in the Sarah
Desert were the Sun would appear in the sky at about ten in the morning as
a white disk. The dust scattering being so great that the sun's disk could
not be seen before that time. The sky there was far from blue again do to
the dust.
        This has been an interesting discussion, even if we are going overboard. I
especially like Daniel's description of scattering. Hecht argues that
reflection and refraction are just a combination of scattering and
interference.

Gary


>    This discussion on sky color has been interesting and informative.
> I have a similar question whose answer I thought I knew, but now
> have some doubt about.
>
>    This summer I traveled to Alaska where I was able to land by
> helicopter on the surface of a glacier.  I had seen the faces of glaciers
> by ship and observed the blue color of the face, but on the top the color
> of the ice in shallow crevasses was spectacular.  It almost seemed as
> though a can of blue paint had been poured into the crevasse.  These
> crevasses were not deep, in most cases being no more than five or six
> meters.
>
>    Now, the standard explanation I have read for the blue color holds that
> the intense pressure due to the weight of the glacier somehow changes the
> crytalline structure of the ice.  Another explanation claims that glacier
> ice is no different from ordinary ice - there's just more of it.  Yet even on
> the surface (where, presumably, the pressure is small) the blue color is
intense
> and even small pieces of ice are blue.  I saw an iceberg that had melted
> away until a very thin sheet jutted up out of the water.  Even though it was
> almost transparent, it was also blue.
>    Thanks,
>    Wes Davis

Gary Karshner

St. Mary's University
San Antonio, Texas
KARSHNER@STMARYTX.EDU