The blue mystery and the Raman effect


 

sky_and_sea

An elementary school level physics question: Why is the sky blue? You may answer that the sunlight gets scattered by the air-molecules and according to Rayleigh scattering theory, the blue light gets scattered the most (the scattering intensity proportional to 1/\lambda^4, implying that it’s high for small wavelength \lambda such as the blue light).

Now the immediate question comes: Why does sea look blue too?

Many school text-books explain in a simple and obvious way: Since the sky is blue, it’s reflection makes river or sea appear blue.

Though seems pretty convincing, the answer is incorrect and this week Tuesday Prof. Chandrabhas Narayan of JNCASR gave a beautiful lecture on Raman effect and how it becomes responsible for the blueness of sea.

He showed a few slides where all water resources don’t appear blue even though they are well-exposed under the sky. He explained in the following way. The Rayleigh scattering is an elastic scattering, where light just get deflected after confronting a molecule, but doesn’t change its wavelength. But there could be inelastic scattering processes due to vibrational modes of a molecules.

In an elastic scattering (Rayleigh one) an electron of a certain energy level inside an atom or a molecule absorbs energy from the incident photon and jumps to a higher energy state (called the virtual level) for a while and then emits the photon of the same energy (hence same wavelength too), and goes back to its original energy state.

However, in an inelastic scattering process, the electron can absorb the energy from a photon and then after reaching the virtual level it may decide to sit on one of its vibrational excited energy level by emitting a light with lower energy (hence higher wavelength). Such an emission line is known as the Stokes lines and the whole scattering process is known as the Raman Effect, discovered by C. V. Raman in 1928, who was a Palit Professor in Calcutta (His discovery helped him to get the Nobel prize very shortly, in 1930.).

The emission can go in the opposite direction as well (vibration level \rightarrow virtual level \rightarrow ground state of the atom/molecule ) and then the lines are called anti-Stokes lines.

Due to this Raman effect the water molecules in sea scatter the white sunlight to wavelengths that mostly fall in the blue regime of light spectra and hence sea appears blue.

Chandrabhas gave a long introduction (entertaining and answering several questions from the audience) of the Raman effect and its special importance in comparison to other spectroscopic measurements.

He said that it’s not necessary that sea or any water resource has to be blue in color (e.g. it can be turquoise) and the color depends on the depth of water as well.

He also mentioned that though Raman effect is very weak (out of 10^9 photons, 1 photon participate in Raman scattering), a newly developed experimental probe called the Surface Enhance Raman Spectroscopy (SERS) can be very useful to characterize nanostructures and for biological fingerprint such as in DNAs and proteins. At the end Chandrabhas mentioned the controversies related to the real discoverers of this effect (e.g. Russians never liked to call it Raman effect, rather they preferred to say Mandelstam effect) and also the credit might have gone to Krishnan, one of Raman’s PhD student.

Prof. Chandrabhas’ talk can be found here.

Now may not be referred as a quiz question, but can you help the child to quench her curiosity?

Some readworthy links:

Raman’s 1921 Nature paper criticizing Rayleigh’s theory for the color of sea
Raman and Krishnan’s 1928 Nature paper reporting the Raman spectra
Biplab Pal’s article
On Raman-Krishnan controversy by Vigyan Prasar

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9 thoughts on “The blue mystery and the Raman effect

  1. violet rays scatters most…and the sky is violet also to a little extent but may be as the next spectra is ultraviolet it effects on it..or may be our sense perception…!!
    well,I don’t know very well..I just guessed it..:D
    do you know???

    • Well, I personally think the issue is still debatable. There are two existing and widely-believed answers.

      1. The violet part of the sunlight are mostly absorbed at high atmosphere.
      2. Our eyes have three cone cells in the retina: red, green, and blue, which are responsible for color vision. Now red and green cells can sense most of the part of the light spectrum, whereas the blue one is limited in the blue regime. Though all three cones response to the color, altogether violet and indigo becomes less sensitive due to the smaller intensities of the response of the cone cells in that
      regime of the spectrum. So overall blue predominates.

      See this: http://tingilinde.typepad.com/.a/6a00d83451b54669e20168ebda37e1970c-pi

      So your second guess is quite close to the answer.

      However, I would prefer to remain still skeptical since we can clearly distinguish the ‘violet’ in a rainbow.

      Maybe if we someday talk about how the 3D glasses work (one glass blue and the other red), it may help us to understand more about our vision.

  2. I had asked the question to D.S. why sky is blue and not violet??
    He said above reasons and also said that sun generates electromagnetic waves within the range of visible light just according to the graph you showed(i.e. describing how our eyes are sensitive to colours) and green is generated mostly(somehow like as it is sensitive to our eyes too) and violet is least generated wave..!!
    so that’s another reason why there isn’t much violet in atmosphere..!!
    he asked me another question that why hills seem blue from a distance whether they are green due to trees on it..
    I said due to scattering of light this happens,he said I pointed out it correctly but still there is more story..:p cause scattering can’t change colour,is it??

  3. Clap my hands! Thank you for such valuable information. I need to explain Raman and I think they way that you have explaine it is one of the best that I have found. So, thank you for sharing it.
    Best

  4. Updates : Well, the post was made by myself on February 2013. Later Veritasium and MinutePhysics have uploaded in May ’13 and Nov ’14 explaining why sky doesn’t appear violet to us. Both talked about the sun’s spectra which has a very little violet contribution compared to blue wavelength. MinutePhysics goes to a more generic explanation in terms of the chromaticity diagram (which also connects to the perception of colors by human eyes, check : http://bit.ly/1qXCkxX) and sun as a hot object (almost a black body) is restricted to a particular color zone in the chromaticity diagram, where depending on varying temperature in the sun, we receive radiations ranging from red to blue (note blue is hotter than red, which is just the Wein’s displacement law), and there again violet part is very insignificant.

    CIE 1931 Chromaticity space (Source : Wikipedia)

    You can check out the videos here : https://www.youtube.com/watch?v=ro2MmmdARrs (Veritasium) , https://www.youtube.com/watch?v=R5P6O0pDyMU (MinutePhysics).

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