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February 1928: Raman scattering discovered

C.V. Raman
Photo courtesy of the
Emilio Segré Visual Archives/AIP

C.V. Raman

On a boat trip back from England in 1921, Indian physicist C.V. Raman, considering the question of why the sea is blue, got started on a line of research that would lead to the discovery in February 1928 of a new scattering effect, now known as  the Raman effect, which is important in physics and chemistry.

C. V. Raman (his full name, Chandrasekara Venkataraman, was shortened during his school years) was born in November 1888 in southern India. His father was a lecturer in physics and mathematics. In 1902, at age 13, Raman entered Presidency College, Madras. He received his bachelor’s degree in 1904 and master’s degree in 1907. As a student he did research in optics and acoustics. 

He loved science, but saw few career opportunities for a scientist in India, so after receiving his degrees he took the financial civil service exam and went to work for the government as an Assistant Accountant General. But he didn’t give up science–in his spare time he carried out experiments at the Indian Association for the Cultivation of Science at Calcutta. His research included studies of the physics of the violin as well as stringed Indian instruments and Indian drums. He also became known as a good lecturer, offering popular science lectures to the public.

In 1917 Raman was offered the Palit Chair of Physics at Calcutta University. In order to take the position he had to leave his prestigious and well paid civil service job, but he was finally able to devote himself full-time to science.

In 1921 he made a trip to England, where he met with distinguished British physicists. He returned to Bombay aboard the S.S. Narkunda, leaving England in September 1921. That was when he began contemplating the striking blue color of the Mediterranean Sea. Lord Rayleigh, who had previously explained the blue color of the sky as due to elastic scattering of light by molecules in the atmosphere, had suggested that the blue color of the sea was simply a reflection of the sky. Raman wasn’t so sure this was the case.

Aboard the ship, Raman had carried with him some simple optical equipment, including a pocket- sized spectroscope and a prism. He used these to examine the water and became convinced that water molecules could scatter light just as air molecules could. He sent a letter to Nature as soon as he got to port in Bombay.

Inspired by his insight aboard the ship, when he got back to his lab in Calcutta he and his research group embarked on a new line of studies on light scattering effects.

In 1922 Arthur Compton had found that X-rays could lose energy, and thus shift to longer wavelengths, in inelastic collisions with electrons. Raman believed that something similar to the Compton Effect could be demonstrated with visible light scattering inelastically off molecules.

Raman and his research group in Calcutta set up some simple experiments to look at the scattering of light in various liquids. As a light source they initially used sunlight, which was abundant in Calcutta. Using a colored filter they separated out blue-violet light, which then scattered off the target liquid. They used yellow-green and other colored filters to visually detect a change in color of the scattered light. The effect is weak and difficult to see, so they soon realized they needed a more intense light source. The research center acquired a seven inch telescope, which Raman used to concentrate sunlight for their experiments. Even with the fairly simple setup, they were able to observe a shift in the color of light scattered by many different liquids.

In February 1928 Raman observed that the scattered light was polarized, which distinguished the new scattering effect from fluorescence. He and colleague K.S. Krishnan sent off a short paper to Nature titled “A New Type of Secondary Radiation,” in which they reported having examined sixty different common liquids and observed the new scattering effect to some degree in all of them. Shortly afterwards Raman measured the exact wavelengths of the incident and scattered light using a spectroscope, and  presented the quantitative results in a lecture to the south Indian Science Association in March and in the Indian Journal of Physics.

Other researchers at the time had also been investigating light scattering effects. For instance, Russian physicists Grigory Landsberg and Leonid Mandelshtam in 1925 began looking at light scattering in quartz. They were at first hindered by poor samples of quartz with many impurities, but by 1928 they managed to get a pure enough sample of quartz, and in February 1928 they independently observed the same scattering effect that Raman had found. They reported their results in at a conference in Moscow in April.

Physicists quickly recognized the importance of the Raman Effect. It provided another verification of quantum theory, and was useful in the study of vibration and rotation of molecules. Within a few years chemists were commonly using Raman spectroscopy for chemical analysis.

Raman received the 1930 Nobel Prize for the discovery, as well as numerous other honors. He was knighted by the British Government in 1929. He later became director of the Indian Institute of Science and established the Raman Research Institute in Bangalore. He died in 1970. In honor of C.V. Raman and his discovery, India observes a national science day on February 28 each year.

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