Recalling C V Raman

Raman Effect
Sisir K Majumdar

Chandrasekhara Venkata Raman was born on November 7, 1888 and died on November 21, 1970 (aged 82). The work of this Indian physicist was influential in the growth of Science in India. He was awarded the Nobel Prize in Physics in 1930 for the discovery that when light traverses a transparent material, some of the light that is deflected changes in wavelength. This phenomenon is now called Raman Scattering and is the result of Raman Effect. Studying the scattering of light in various substances, in 1928 he found that when a transparent substance is illuminated by a beam of light of one frequency, a small portion of the light emerges at right angles to the original direction, and some of this light is of different frequencies than that of the incident light. These so-called Raman frequencies are equal to the infrared frequencies for the scattering material and are caused by the exchange of energy between the light and the material.

On February 28, 1928, through his experiments on scattering of light, Raman discovered the Raman effect and the research was published in the   Indian Journal of Physics, Calcutta ("A New Radiation", Vol 2, P 387-399, March 31, 1928). The work was simultaneously published in the Nature, London (A New Type of Secondary Radiation", Vol. 121, P 501, March 31, 1928).

It is the change in the wavelength of light that occurs when a light beam is deflected by molecules. When a beam of light traverses a dust-free, transparent of a chemical compound, a small fraction of the light emerges in directions other than that of the incident (incoming) beam. Most of this scattered light is of unchanged wave length. A small part, however, has wavelengths different from that of the incident light; its presence is a result of the Raman Effect.

Raman scattering is perhaps most easily understandable if the incident light is considered as consisting of particles or photons (with energy proportional to frequency), that strike the molecules of the sample. Most of the encounters are elastic, and the photons are scattered with unchanged energy and frequency. On some occasions, however, the molecules take up energy from or gives up energy to the photons, which are thereby scattered with diminished or increased energy, hence with lower or higher frequency. The frequency shifts are thus measures of the amounts of energy involved in the transition between initial and final states of the scattering molecule.

The Raman Effect is feeble; for a liquid compound the intensity of the affected light may be only 1/100,000 of that incident beam. The pattern of Raman lines is characteristic of the molecular species, and its intensity is proportional to the number of scattering molecules in the path of the light. Thus, Raman spectra are used in qualitative and quantitative analysis.

It was instantly clear that this discovery was of immense value, it gave further proof of the quantum nature of light. Raman spectroscopy came to be based on this phenomenon.

Raman was president of the 16th session of the Indian Science Congress in 1929. He was conferred a Knighthood (1929) by the then British Colonial Government, and various medals and hononary doctorates by various universities. He was awarded the 1930 Nobel Prize in Physics "for his work on the scattering of light and for the discovery of the effect named after him". He was the First Asian and First non-White to receive any Nobel Prize in the sciences. Before him Rabindranath Tagore (1861-1941), first Asian-first Indian-first non-white, had received the Nobel Prize in Literature in 1913. He was elected Fellow of the Royal Society, London early in his career in 1924. In 1947, he was appointed the first National Professor by the Government of independent India. He was awarded the Ranklin Medal in 1941, Lenin Prize in 1957.

Raman discovered the quantum photon spin in 1932, which further confirmed the quantum nature of light.

Raman also worked on the acoustics of musical instruments. He worked out the theory of transverse vibration of bowed strings, on the basis of superposition velocities. He was also the first to investigate the harmonic nature of the sound of the Indian drums as the table the mridangam.

Raman provided the correct theoretical explanation for the acousto-optic effect (light scattering by sound waves), in a series of articles resulting in the celebrated Raman-Nath theory. Modulators, and switching systems based on this effect have enabled optical communication components based on laser systems.

Raman carried out experimental and theoretical studies on the diffraction of light by acoustic waves of ultrasonic and hypersonic frequencies (published 1934-1942), and those on the effects produced by X-rays on infrared vibrations in crystals exposed to ordinary light.

In 1948, through studying the spectroscopic behaviour of crystrals, approached in a new manner fundamental problems of crystal dynamics. He dealt with the structure and properties of diamond, the structure and optical behaviour of numerous iridescent substances (labradorite, pearly feldspar, agate, opal, and pearls). Among his other interests were the optics of colloids, electrical and magnetic anisotropy, and the physiology of human vision.

He was a multi-facted scientific genius.

Raman started a company called Chemical and Manufacturing Co Ltd, in 1943 along with Dr Krishnamurthy. The Company during its 60-year history established four factories in Southern India. In this regard, he followed the footsteps of another eminent Scientist-Acharya Praffulla Chandra Roy [1861-1944) who founded the Bengal Chemical and Pharmaceutical Works in Calcutta in 1893.

Raman was awarded the BHARATRATNA-the highest civilian honour in India, in 1954. India Celebrates NATIONAL SCIENCE DAY on February 28 every year (Since 1987) to commemorate the discovery of the Raman Effect in 1928.

Frontier
Vol. 45, No. 3, July 29-August 4, 2012