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Snow and ice
Icebergs floating near Seward, Alaska. Blue to bluegreen hues are scattered back when light deeply penetrates frozen waterfalls and glaciers.
What causes the blue color that sometimes appears in snow and ice? As with water, this color is caused both by the absorption of red and yellow light (leaving blue). The absorption spectrum of ice is similar to that of the liquid except that hydrogen bonding causes all peaks to shift to lower energy -- making the color greener. This affect is augmented by scattering within snow, which causes the light to travel an indirect path, thus providing more opportunity for absorption.
From the surface, snow and ice present us with a uniformly white face. This is because most all of the visible light striking the snow or ice surface is reflected back without any particular preference for a single color within the visible spectrum. The situation is different for that portion of the light which is not reflected but penetrates or is transmitted into the snow. As this light travels into the snow or ice, the ice grains scatter a large amount of light. If the light is to travel over any distance it must survive many such scattering events, that is it must keep scattering and not be absorbed. The observer usually sees the light coming back from the near surface layers (less than 1 cm) after it has been scattered or bounced off other snow grains only a few times and it still appears white.
Iceberg calved from Alaska's Le Conte glacier. This iceberg is more icy and clear than the one above, so it appears more blue. The larger grain sizes of bubbly ice allows deeper penetration of incident light and a reflected hue that can vary from blue-green to blue depending on the color of the surface which underlies the ice.
Deep blue surface is typical for Jupiter's icy moon Europa. The smooth blue is composed of almost pure water ice. Brown linear ridges may be remnants of cryovolcanic activity, possibly mineral salts in a water ice matrix. Scale: approx 50 km wide.
Deeper in the snow, the preferential absorption of red begins to become noticeable. Just like with water, more red light is absorbed compared to blue. Not much more, but enough that over a considerable distance, say a meter or more, photons emerging from the snow layer tend to be made up of more blue light than red light. Typical examples are poking a hole in the snow and looking down into the hole to see blue light or the blue color associated with the depths of crevasses in glaciers. In each case the blue light is the product of a relatively long travel path through the snow or ice. So the spectral selection is related to absorption, and not reflection as is sometimes thought. In simplest of terms, think of the ice or snow layer as a filter. If it is only a centimeter thick, all the light makes it through, but if it is a meter thick, mostly blue light makes it through. This is similar to how coffee often appears light colored when poured, but appears much darker when it is in a cup.
Does glacier ice last longer in drinks? Yes, a little, because the ice crystals are larger. Crystals melt from the outside and large crystals expose less surface area per unit volume of ice; therefore, ice with larger crystals melts more slowly.
