Water, ice (and fire)

This is about the colour of water, a subject that interests me. Both pure water - seen against a light-coloured background - and transparent (few air bubbles) ice have a beautiful pure blue colour when seen in sufficient depth. The layer of either ice or water needs to be at least a metre thick before the colour becomes clearly apparent. The picture here - taken by me in 2006 from a kayak near the landward end of Glacier Bay in Alaska - shows the beautiful pale, transcucent blue of the glacier ice as it calves into the sea.

Why is the water blue and not transparent like pure clear glass? It is not due to an impurity, it is due to an intrinstic property of the water molecule itself: one oxygen and two hydrogen atoms (H_2O, where the "_" indicates that the following character is a subscript). The hydrogen atoms - about 16 times less massive then the oxygen atom - can vibrate in a number of different ways. The hydrogens sit next to the oxygen, not in a straight line but bent with an angle of almost 105 degrees separating them. They can vibrate along the line joining them to the oxygen, either symetrically (both together) or antisymetrically (like a boxer punching with one arm followed by the other): these are called 'stretch' modes. The two hydrogens can also 'bend', changing their separation angle around 105 degrees. There are other vibration modes but they are not really relevant to the colour.

Because the hydrogen atoms are so light, their vibrations are very fast and, unlike most other molecules, extend from the infrared into the frequencies occupied by red, yellow and green light. Combinations of different vibrations produce a series of 'absorption bands' extending from the infrared down through the visible red and, with decreasing strength, into the green part of the spectrum. The result of this is that deep water or thick ice has much of its red and eventually yellow and green light absorbed, leaving the blue light to travel without hindrance.

The rather complex-looking diagram here shows what these bands look like in water vapour, liquid water and ice. They are all measurements made with my JAZ spectrometer.

Focussing first on the liquid water (the pale blue line), you can see that a few cm of water is perfectly transparent (100%) in the visible spectrum (up to 700nm) and only as we look into the near infrared does it begin to be absorbed strongly. This measurement was made through a path of 5.5cm of water but, with a metre or two, the absorption would extend further towards the green - to the left of the diagram. Ice (the brown line) is similar in its effect although the dips are shifted a bit towards the red - basically because the vibrations of the hydrogen atoms are inhibited by the neighbouring water molecules in the solid. The positions of the water bands are marked by the light blue 'X' and '+' symbols (the difference being due to the different combinations of vibrations).

The vapour bands (seen in the dark blue spectrum) are furthest to the blue (left) because the hydrogen atoms are less affected by neighbouring molecules in the gas phase. They are marked with the dark blue circles and triangles. This water vapour spectrum was obtained at sunset when the Sun has to shine through a very long path of air. This maximises the strength of the absorptions which are due mostly to water but also to other gases such as oxygen and ozone. The features in the solar spectrum as well as the ozone absorption have been removed from this 'telluric' spectrum (see: www.eso.org/sci/publications/messenger/archive/no.143-mar... ). The ozone actually has a huge effect on the colour of the twilight sky: but that is another story! The molecular oxygen bands have been marked with the green squares. On the left hand side, there are also some quite strong absorptions due to the O_4 'dimer': a loose and temporary coupling between two oxygen molecules. These dimer bands are shown overlayed on the blue spectrum.

Water is seen not only in absorption. In a hydrocarbon gas flame, here from a butane kitchen blowtorch, the same vapour bands are seen in emission (the red spectrum). Water emission bands can also be seen in the spectra of comets as they near the Sun. The positions of all the 'flame' bands are marked with the red diamonds.

The final spectrum in this diagram is the transmission of an emerald pebble (green line) . The emerald crystal structure contains 'channels' which are big enough to contain almost 'free' water molecules which reveal themselves here as the dip at a wavelength of 960nm.

The water molecule behaves in a wonderfully complex way, giving water the most amazing and life-enabling properties. These spectra give a mere glimpse of the full set of complexities!

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