Thirty years ago, 28 February was announced as National Science Day in India as the world-renowned Raman Effect was invented on that day in 1928. The object of observing National Science Day is not only to honour CV Raman but also to inspire Indian scientists and technologists.
Raman’s father Chandra Sekhar Ayer was a professor of physics and mathematics while his mother came from a conservative Sanskrit family. An exceptionally high intellect was inherited from his parents and Raman graduated at 16. At that time, there was hardly any scope for scientific research and so he appeared for a competitive examination for entry into the finance department. As usual, he secured the top position and was selected as assistant accountant general, finance, Government of India. He was only 19 and took up the job in Calcutta where he stayed for the next 10 years.
Once on his way to office, Raman noticed the Board of Indian Association for Cultivation of Science. He expressed his desire to conduct research to Amritalal Sarker, then director of the institution, which was established in 1878. It was decided that only Indian scientists would be accepted and Raman was cordially invited to start his research work — he continued that from 1907 to 1917. Next, he became the departmental head of physics at the University of Calcutta.
In 1921, Raman received his first invitation from Europe. In those times, ship was the only mode of transport to travel long distances and on that long sea voyage, he got curious as to why various shades of blue were spread in the sky and sea. The sky is a void and water is colourless, but why do they appear blue? This curiosity made him begin research work on the scattering of light waves through different media like solid, liquid and gas.
The great scientist Lord Raleigh had the idea that scattering of light waves through air particles makes the sky azure and the greenish blue colour of sea water is caused by reflection of that sky in sea water.
Raman did not accept the theory. He conducted research about different colours of the sky — at dawn, twilight and other times throughout the day and concluded that same theory is applicable to both sea water and the sky. In both cases, sun rays are scattered in different wave lengths through air and water particles. Through various experiments Raman concluded that any liquid contains minute particles and because of them blue colour is scattered. The seven visible colours of the spectrum are commonly known as “Vibgyor” and are set from smaller to larger wave lengths. After scattering, rays of smaller wave lengths like violet, indigo, blue and green are spread more widely than yellow, orange and red, which have larger wave lengths.
Though violet has the shortest wave length, the sky looks blue, not violet. This is because, the rays penetrating through the atmosphere and reaching Earth contain mostly blue colour. Also, our eyes are attracted more to wave lengths of blue. During sunrise or sunset, the sun lies close to the horizon and at those times, the “yor” portions are scattered more. As the density of air becomes lesser, the scattering of Vibgyor lessens and sky turns dark.
In oceans, sun rays are scattered through water particles almost at an angle of 30 degrees. In that range, scattering of sun rays though water particles become 150 times wider than scattering of sun rays through air particles (omitting dust particles). Therefore ocean water does not look like the sky but appears as greenish blue.
Light appears in two features — waves and photon particles. When a monochromatic ray passes through pure matter, most photon particles will pass directly. A little portion will be scattered in different directions after interaction with the particles of the medium. Raleigh came to the conclusion that in a liquid medium, a tiny portion of the scattered ray travels in the same wave length as the incident ray but only changes direction. Later, more investigations showed that the scattered ray contains photon particles of frequencies varied than incident ray. This variation in frequency is manifested due to the specific nature of the medium’s particles.
When a photon particle is incident on a molecule of the medium, it can interact with an electron either in the ground state or high levels. Number of electrons in the ground state always exceeds the number of electrons in higher energy levels. Therefore the possibility of interaction with electrons in the ground state is always higher. For this reason, electrons gain energy from photon particles and increase their energy levels. This restless condition of electrons persists for a very short interval and therefore there are greater possibilities for repeated scattering of photon particles in same energy and same frequency levels. This is known as Raleigh Scattering.
On the other hand, in every medium, a limited number of electrons always exist in high energy levels. An electron in the ground state gains the energy of the incident photon particle and rises to a higher energy level. Then the energy of the scattered photon particle will have the difference of the energy of the electron and the energy of the incident photon. Similarly, if an electron in high energy levels gains energy of the incident photon then the scattered photon will possess the sum of the energy of both. This is the simplest way to understand the Raman Effect.
This transfer of energy is dependent on the vibration of particles and temperature. When the temperature diminishes, particles of the medium will absorb energy from the incident photon and energy of the scattered beam diminishes. Therefore frequency diminishes and wavelength increases. As temperature rises, the particles of the medium supply energy to the incident photon. The frequency increases, thereby causing a decrease in wave length. In either case, the wave length of the scattered beam differs from that of the incident beam.
The funniest thing is that the apparatus in this illustrious invention by Raman contained a mirror to reflect the sun’s ray, a lens; a pair of complementary glass filters to obstruct the selected paths of rays, a flask full of benzene and a pocket spectroscope. At the time, the total cost was barely Rs 200! And he was awarded the Nobel Prize in Physics in 1930.
Its full name is the Raman-Krishnan Effect as his brilliant student KS Krishnan helped him by laboriously conducting research over several years. In 1933, Raman became the director of the Indian Institute of Science in Bangalore and the next year, he established the Indian Academy of Science. From 1940, he started research on the scattering of light in crystals, gem stones, pearls, corals and diamonds. He was awarded the Bharat Ratna in 1950. Owing to his affinity for roses, Raman maintained a rose garden where he was buried in 1970.
Raman never went abroad for higher education and conducted all his research in India — he tried to motivate scholars in the same way. Of course, now the scenario differs greatly owing to globalisation and as such is quite necessary. But one only wishes that National Science Day is observed every year in a big way as a mark of respect to CV Raman and the ideals he lived by.
The writer is a former lecturer in Applied Mathematics, Maharajadhiraj Uday Chand Women’s College, Burdwan.