Introduction

All the colors in the universe originate from fifteen physical causes. These fundamental physical causes occur over and over again throughout nature. Some of these causes include scattering, refraction, dispersion, interference and transitions and rotations. The intrinsic blueness of water is the only example from nature in which color originates from vibrational transitions.

Background

Almost all materials owe their colors to the interaction of visible light with electrons. Whether the colors originate from resonant processes such as absorption and reflection, or non-resonant interactions including Rayleigh scattering and interference, the photons of light interact exclusively with the electrons of the matter, producing its color. Water is the only example found in nature where its color is caused by vibrational transitions.

Water is unique among molecules as it is full of hydrogen bonds. The hydrogen bonds in water make water more colorless than gas. The role of hydrogen bonding can be determined by comparing water in its gaseous and liquid phase. When comparing the vibrational transitions between gaseous and liquid water, there is a shift to lower energy induced by hydrogen bonding. As the temperature of water is increased, the number of hydrogen bonds decreases. The absorption of red wavelengths is attributed to high vibrational transitions that have been shifted to lower energy levels by hydrogen bonding. This absorption of red wavelengths is responsible for the blueness of water. The hydrogen bonding in water causes the stretching frequencies between water molecules to lower values, causing water to be clearer. If there were no hydrogen bonds, the blueness of the water would be more intense. While one may hypothesize that other hydrogen containing solids and liquids possess traces of blue due to similar causes, water is the only chemical substance (along with ice) observed in pure form and in large quantities in nature where coloration by weak vibrational transition is detectable.

There are three main factors that affect the color one sees in water. First, particles and solutes in the water can absorb light, altering its blueness. The presence of green algae in rivers and lakes, for example, will create a blue-green color. The second factor is the scattering of light due to particles suspended in the water. This light scattering in water is required so that the blue light caused by vibrational transitions can return to the surface to be observed. The deeper the water, the more saturated the color. Finally, the surfaces of bodies of water reflect light. The reflection of the blue sky, for example, makes rivers and lakes appear bluer.

Pure water and ice have a pale blue color. This color can be seen best at tropical white sand beaches as well as in ice caves in glaciers. Because water absorbs wavelengths in the red end of the spectrum, one sees blue when light has passed through several meters of water. The absorption of light in ice is similar to that of liquid water. However, the hydrogen bonds in ice cause all peaks to shift to lower energy levels, making the ice appear slightly greener than liquid water. From the surface, snow and ice appear white. Almost all visible light striking the snow or ice surface is reflected. However, the light that passes through or is transmitted into the snow undergoes scattering by millions of grains of ice. Deeper in the ice and snow, the red wavelength absorptions become more noticeable, as the ice or snow appears blue.

Observations and Data

	Vibrational transitions are known to be the cause of the blueness of water because "heavy water" (D2O) has the same wavelength absorption curve as water but shifted to longer, slower frequencies as seen in the chart on the left. The picture above compares two tubes, the one on the left containing water and the one on the right containing heavy water. The heavy water has no color as its vibrational transitions are slower than that of pure water.

Conclusions

While there are only 15 physical causes responsible for all colors in nature, water is the only known substance whose color is derived from vibrational transitions. The absorption of red wavelengths from hydrogen bonding is the source of water’s intrinsic blueness. While other factors such as light scattering and absorption by particles as well as reflections by water’s surface may change the observed color of water, pure water is a pale blue.

Links

http://www.dartmouth.edu/~etmsfer/water.htm

http://webexhibits.org/causesofcolor/5.html

Other References

Nassau, Kurt. The Physics and Chemistry of Color: The Fifteen Causes of Color. 2nd ed. New York: John Wiley & Sons, Inc, 2001.

Williamson, Samuel J. and Herman Z. Cummins. Light and Color in Nature and Art. New York: John Wiley & Sons, Inc, 1983.

Acknowledgements

I would like to thank Dr. Bordley for his guidance, as well as Jurnell Cockhren and Julie Adams for helping to create this website.