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Raman Effect

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VoicenData Bureau
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The phenomenon of "Raman Scattering" was discoveredin 1928 by Indian physicist, Chandrasekhar Venkata Raman, popularly known as CVRaman, who received the Nobel Prize for Physics in 1930, and was later awardedthe "Bharat Ratna". The first experiments demonstrating RamanScattering were conducted with extremely limited or, if one can say, rathercrude set-up: Sunlight providing the light source, a telescope collecting thescatter from the sample, and Raman’s eyes serving as the detector.Complementary coloured glass filters were used to select the colour of lightstriking the sample and to block out the light that was elastically scattered(of same wavelength as the incident light) from the sample.

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When light is transmitted through matter, it is scattered inrandom directions. Most of the scattered light is of the same wavelength (orfrequency) as the incident light. This is known as "Rayleighscattering" and is considered elastic as the scattered photon has the sameenergy as the incident photon. Some light is inelastically scattered–scatteredphoton having higher or lower energy than incident one–at a differentwavelength (or frequency). This is called "Raman Scattering". Thisshifting of frequency is also called "Raman Effect", which arises whenthe incident light excites molecules in the sample, which subsequently scattersthe light.

Typically, only one part in a thousand of the total intensityof incident light is Rayleigh scattered, while for Raman scattering this valuedrops to one part in a million (0.0001 percent). Thus, Raman effect being quiteweak, it is a major challenge in Raman spectroscopy to attenuate the light thatis elastically scattered in order to detect the inelastically scattered Ramanlight.

"Raman Scattering" results from the molecule changing its molecularmotions. The energy difference between the incident light and the Ramanscattered light is equal to the energy involved in changing the molecule’svibrational state, and is called the "Raman shift". If the initialbeam is sufficiently intense and monochromatic, a threshold can be reachedbeyond which light at the Raman frequencies is amplified, builds up strongly,and generally exhibits the characteristics of stimulated emission. This iscalled the stimulated or coherent Raman Effect. A device illustrating thestimulated Raman Effect is sometimes called a Raman laser.

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