Ice Blues

Ice Blues ~ A deep blue hole in a light blue berg floating near the Upsala Glacier, Argentine Patagonia. Contrast the blues with the similarly lit snow on the mountains.

The lower image shows more blue ice on Glacier Perito Moreno also in Argentina.

Both images by Mario Freitas (site) of Universidade Tecnológica Federal do Paraná. ©Mariao Frietas, shown with permission.

Pure ice in sufficient thickness is blue. It is most transparent to blue-violet light of wavelength 380 – 420nm and progressively less so to longer wavelengths and especially red. Light shining through it is therefore increasingly blue.

However, the absorption is not quite that straightforward because glacier and polar ice is highly compacted snow and the individual grains scatter the light. Light eventually transmitted has arrived via many convoluted ray paths and absorption – scattering events.

Multiple scattering by ice grains confers the overall external white appearance to ice but that too is modified by light that has penetrated slightly and before re-emerging has had its longer wavelengths absorbed.

Remember too that shadowed ice is often only illuminated by the blue sky.

Early polar explorers expected to see only white. Instead they came upon wondrous sights of ice of all hues, blues, mauves, greens, reds and browns. Near white objects are very sensitive to the incident sunlight and sky colours.

But many ice colours are intrinsic. The blues mostly come from absorption, other colours from trapped micro-organisms and mineral grains. Even embedded soot from our combustion processes modifies the optical properties of polar ice. The absorption spectrum at right is derived for pure ice and the transmittivity of natural ices is usually much lower.

Add to all that the colours from birefringence and thin film interference and there is much to ice for us to see.