The Raman Effect

The beginning of the 20th century marked significant breakthroughs in physics across the world. A great deal of work was conducted to understand the properties and dual nature of light. The light-scattering phenomenon was studied earlier by an English physicist, Lord Rayleigh. When a light ray travels through a medium, it gets deviated due to the particles present in the medium. This phenomenon is known as the scattering of light. According to the Rayleigh scattering, the energy and the wavelength of the light ray before and after scattering remain the same. This is called elastic scattering. This phenomenon explains the blue color of the sky- shorter wavelengths (blue and violet) are scattered by the atmospheric particles more strongly than the longer wavelengths (orange and red). The reverse of this happens during the sunset when the skies appear orange-red due to the scattering of longer wavelengths. 

Around the same time, CV Raman and his student, KS Krishnan, independently studied the light scattering phenomenon and conducted a few experiments at the Indian Association of Cultivation of Sciences (IACS), Calcutta. However, what they observed was different from the Rayleigh scattering effect. The energy and wavelength of the scattered light differed from the incident radiation. This is inelastic scattering. CV Raman attributed this to an analogue of the Compton effect, which explains the interaction between high-energy photons in X-rays that deviate in wavelength after interacting with charged particles like electrons. But the novelty about Raman scattering lay in the fact that it was observed even in ordinary light rays, not just the high-energy X-rays. They published this finding in Nature through an article titled ‘A new type of secondary radiation’. Thus, in an ordinary light, along with the regular elastic Rayleigh scattering, there is also the presence of the Raman scattering phenomenon, which gives a wavelength-shifted scattered light ray along with the same-wavelength scattered light. This makes the energy associated with the incident and the scattered ray different. These findings were published on the 28th of February 1928, and Sir CV Raman was awarded the Nobel prize, the highest recognition for a scientist worldwide. 

Since then, National Science Day has been celebrated annually in memory of the Raman effect. Although the Raman effect might appear as a fundamental discovery, it has a wide range of applications in modern science. Raman spectroscopy is an instrumentation technique widely used in the chemical analysis of compounds and materials. It works on the principle of the Raman effect. The key component of the Raman spectrometer is a laser that helps in inelastic scattering of the sample to be tested i.e. the energy of the scattered light is different from the incident one. The peaks in the resulting spectrum (caused because of the scattered light radiation) correspond to specific vibrational modes within the sample's molecules, allowing for identification and characterization of the sample. The spectrum obtained through Raman spectroscopy is unique for every molecule and thus can be used for its identification. A Raman spectrometer is now commonplace in research laboratories and chemical industries worldwide. Thus, a fundamental, natural observation could be quantified for real-world application and usage. 

©Neha Kanase