ARTICLE: ON THE EVE OF NATIONAL SCIENCE DAY, FEB-28
Raman Effect: Re-visited after 85 years
Way
back in 1921 in a return trip from London to Bombay abroad the ship S.S. Narkunda,
an young Professor of Calcutta University was awe struck by the deep blue colour
of the Mediterranean. His restless probing mind was so much captivated by the
blue sea that he started his own experiment abroad the ship itself to verify
the claim of Lord Rayleigh the British
Physicist that the blue colour of the sea is simply the reflection of the
colour of the sky. He was none other
than Chandrasekhara Venkata Raman (1888-1970) who at that time held the Palit
chair of Physics at Calcutta University. During his maiden voyage of fifteen
days his inquisitive mind was in romance with the optical illusion of the
Mediterranean. He spent hours at the deck of the ship with his pocket
spectroscope and a Nicol prism. During his observations Raman was surprised whedcn
he discovered that the blue colour of the sea persist even when the weather
changes and also with it the haze of the sky. This clearly
defy Raleigh's reflection. Raman was very much excited with his observations
and he communicated his thought to the editor of Nature as soon as S.S.
Narkunda docked at the Port of Eden. This was followed by another
correspondence to Nature when the ship finally docked at Bombay as Raman was
convinced that it was scattering of sunlight by water molecules and not the
reflection of the colour of the sky that
is responsible for the blue colour of the sea. Returning home Raman started his
investigation on light scattering by different solids and liquids. His initial
experiments involved a simple mercury arc lamp, a flask of benzene and a pocket
spectroscope. He then designed a low cost quartz spectrograph to make precise
measurement of his experiments and take photograph of the scattered radiations.
In his quest for unveiling the optical property of liquid molecules Raman
passed highly monochromatic light through different liquids and to his surprise
he observed that apart from the incident wavelength the scattered light had
another component with larger wavelength. Raman immediately communicated his
observations to Nature, titled “A New
Type of Secondary Radiation,” where he reported that approximately 60
different liquids had been studied, and all showed the same result—some
scattered light had a different color than the incident light. “It is thus,”
Raman said, “a phenomenon whose universal nature has to be recognized.”
Raman's observation compelled the scientist to re-evaluate all the existing
theories of light. The very first reaction from the western scientist was the
possibility of feeble fluorescence
from impurities present in the liquid. Raman repeated his experiment with ultra
pure liquids and observed the same so called feeble fluorescence which ruled out the claim of western
scientists. Raman than moved to natural light. He used sunlight, focused by a
wide aperture telescope with proper filters as a source and after many hours of
exposure, between 16th February and 28th February, 1928, the signatures of the
scattering with modified lines, were discovered. To distinguish from
fluorescence, the polarization of the lines were determined. It was seen that
the electric field in these lines always satisfied some preferential properties
while no such effects would happen with fluorescence. Unlike fluorescence, it
was a single step process Thus the so called “feeble fluorescence” was no
fluorescence at all and one of the greatest discovery on optical property of
liquid molecules was made. Raman
announced his discovery on 28th February, 1928, and the press carried the news
on the 29th (it was a leap year). On 16th March, 1928, he gave a detailed
report at Bangalore, in his inaugural address to the South Indian Science
Association. The quantitative results of his experiment were first published in
the Indian Journal of Physics on March 31, 1928. Other scientists quickly
understood the significance of this phenomenon as an analytical and research tool
and called it the Raman Effect. Raman effect or Raman scattering also came to
be known as an inelastic scattering of a photon. When light is scattered from
an atom or molecule, most photons are elastically scattered with almost the
same energy (frequency) and wavelength as the incident photons. But a small
fraction of the photons is scattered by excitation. The frequency of scattered
photons is lower than the frequency of the incident photons.
The
significance of this great discovery can be realized from the statement of
Albert Einstein who said," C.V. Raman was the first to recognize and demonstrate
that the energy of photon can undergo partial transformation within matter. I
still recall vividly the deep impression that this discovery made on all of
us.............". . For his discovery Raman was awarded the Nobel
Prize in physics in 1930. He thus became the first Asian to win the Nobel
Prize. After receiving the Nobel Prize Raman said, "When the Nobel award was
announced I saw it as a personal triumph, an achievement for me and my
collaborators -- a recognition for a very remarkable discovery, for reaching
the goal I had pursued for 7 years. But when I sat in that crowded hall and I
saw the sea of western faces surrounding me, and I, the only Indian, in my
turban and closed coat, it dawned on me that I was really representing my
people and my country. I felt truly humble when I received the Prize from King
Gustav; it was a moment of great emotion but I could restrain myself. Then I
turned round and saw the British Union Jack under which I had been sitting and
it was then that I realized that my poor country, India, did not even have a
flag of her own - and it was this that triggered off my complete
breakdown". By the late 1930s the Raman Effect had become the
principal method of nondestructive chemical analysis for both organic and
inorganic compounds. The unique spectrum of Raman scattered light for any
particular substance served as a “fingerprint” that could be used for
qualitative analysis, even in a mixture of materials. Further, the intensity of
the spectral lines was related to the amount of the substance. Raman
spectroscopy could be applied not only to liquids but also to gases and solids.
And unlike many other analytical methods, it could be applied easily to the
analysis of aqueous solutions. It was a ubiquitous technique, giving information
on what and how much was present in a plethora of samples. This method became
even more valuable with the advent of modern computers and lasers. Its current
uses range from the non-destructive identification of minerals to the early
detection of life-threatening diseases
In 1986 on recommendation of the
National Council for Science and Technology Communication (NCSTC), the
Government of India designated February 28 as the National Science Day (NSD) to
commemorate the discovery of Raman Effect. On 28th February 1987 the very first National Science Day was celebrated. The
focal theme of National Science Day – 2013 (NSD-2013)
is “Genetically Modified Crops and Food Security
- Issues and Prospects ”.
The American Chemical
Society designated the Raman Effect as an International Historic Chemical
Landmark in 1998.
What
makes Raman Effect unique is not only its extensive application but also the
fact that it was discovered in pre independent India when science was a pride of the west. Raman's indigenous
equipments proved better than the advanced technology of the west. The Raman
Effect is a very weak effect; only one in a million of the scattered light
particles, or photons, actually exhibits the change in wavelength. This
explains, in part, why the effect was not discovered earlier. Raman with his
inexpensive spectrograph not only detected but also photographed the spectrum
of the scattered light and measured its wavelength. This discovery led to the
development of Raman Spectroscopy, the most efficient and precise analytical
tool today. With the development of the Fourier transform (FT) technique and
the application of computers for data handling, commercial FT-Raman spectrometers
became available in the late 1980s, resulting in a resurgence in the use of the
original Raman Effect. The new Raman spectroscopy has been used to monitor
manufacturing processes in the petrochemical and pharmaceutical industries.
Today after eighty five years scientists are still actively working out the
results and practical applications of Raman’s deceptively simple experiment.
However inspite of a Nobel discovery in the pre independence era itself, Indian
science could not advance much. Raman Spectrometer is today an indispensible
analytical tool for all scientific research but
unfortunately India today imports Raman Spectrometers from abroad.
Compiled by:
DR PALASHMONI SAIKIA
ASSOCIATE PROFESSOR
DEPARTMENT OF CHEMISTRY
DARRANG COLLEGE
TEZPUR-784001
EMAIL: palashms@rediffmail.com
Mob: 9435082506
Fig-1.1
