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 , implying that it’s high for small wavelength 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 virtual level 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 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