Wednesday, April 30, 2014

Blue snow, eternal winter

The Blue Ice of Antarctica, photo by Brendan van Son [1]

In my neighborhood, a clean and newly fallen snow (one of many this winter) showed a blue color in its concavities---an effect similar to the more grandiose photo above. The sky was gray---as in this photo---so it couldn’t be a reflection of the blue of the sky. Furthermore, although I had seen plenty of photographs of blue-looking snow and dismissed them as artifact, this time my direct view denied that excuse.

Why was the snow blue? The main mechanism of the blueness is water’s absorption of long-wavelength visible light, leaving the short-wavelength end of the spectrum to reflect to a viewer.  Incident light finds a long path length through the snow (or ice), followed by deep scattering from air bubbles. The concavities intensify the blueness through multiple reflections.

Water’s absorption of long-wavelength light is familiar to scuba divers, and is responsible for the success of blue-green lasers for undersea communication: red lasers don’t penetrate very far into the water.

Seeing the blue snow reminded me of a remark I made about C. V. Raman in an earlier Hue Angles [2]: “A 1921 trip returning to India from England made him marvel at the blue of the ocean, and to posit that blue as arising from molecular scattering of light by water molecules, not just reflection of the blue of the sky (as Lord Rayleigh supposed).” Soon after his return, Raman wrote a long paper for the Royal Society that said the sea is blue because of the same kind of scatter as Rayleigh attributed to the sky---so the sea generates its own Rayleigh-type scattering [3, p. 35; 4].

To me, the clarity of images seen through even a large path-length of clean water seems inconsistent with scatter---which incurs haze due to random change of direction of the light. Is the blue of the ice cliff and of my small snow-dimples due to molecular scatter per Raman’s thinking in 1922? I don’t think so.

Raman’s fascination with ocean color ultimately led to his receiving the Nobel Prize for discovering the Raman effect. Can the blue of water or snow be explained by the Raman effect? Again, no---and I think he would have agreed. The Raman effect is much weaker than the blue light from the ice (only one in ten million photons is Raman-scattered, which is even less than the one in ten thousand for Rayleigh scattering from the sky) [5].

Given the weakness of the Raman effect, how can it be observed at all? One needs two tricks.

First, to eliminate the reflected light, illuminate a sample with an excitation wavelength and observe it at a somewhat longer emission wavelength. Raman used the 435.8 nm line of a mercury lamp (with other lines filtered out) as an excitation, and then removed the 435.8 nm line with a further filter after the light had interacted with the sample. What was left was the small portion of the light whose wavelength was altered by the sample. Getting the right filters to do this was not easy, especially in Raman’s time and place.

Secondly, to eliminate fluorescence as an explanation, observe whether the wavelength-altered light is polarized. Fluorescence comprises absorption and re-radiation of light, with no memory of the geometry of the incident light.  But Raman-scattered light is polarized [3, 6] so the electric field is perpendicular to the plane of the incident ray, scatterer, and detector.   Furthermore, unlike in fluorescence, the wavenumber (or frequency) shift of Raman-scattered light is independent of the excitation wavelength [5].

So, you can see that the blue color of water, ice and snow inspired much science and some ideas that even now are open to debate. Seven busy years separated Raman’s marveling at the blueness of the ocean and his discovery (1928) of the effect that bears his name.  I recommend Venkataraman’s book [3] for a chronicle of those years.

Returning to the seemingly eternal winter of 2014, I hope that by the time you read this column you will be contemplating the greens and yellows of spring.

1.; for other photos see
2. M. H. Brill, C.V. Raman’s explorations in color science, ISCC News # 441, Sep-Oct 2009, pp. 3-4.
3. G. Venkataraman, Raman and his Effect by (Universities Press, 1995, reprinted 2009).
4. C. V. Raman, On the molecular scattering of light in water and the colour of the sea. Proc. R. Soc. Lond. A 101 (1922), 64-80. [see]
6. C. V. Raman and K. S. Krishnan. A new type of secondary radiation. Nature (London) 121, 501-502 (1928). [see]

Michael H. Brill
(6 March 2014)