Raman, in his broadening venture on optics, started to investigate scattering of light starting in 1919. His first phenomenal discovery of the physics of light was the blue colour of seawater. During a voyage home from England on board the S.S. Narkunda in September 1921, he contemplated the blue color of the Mediterranean Sea. Using simple optical equipment, a pocket-sized spectroscope and a Nicol prism in hand, he studied the seawater. Of several hypotheses on the colour of the sea propounded at the time, the best explanation had been that of Lord Rayleigh's in 1910, according to which, "The much admired dark blue of the deep sea has nothing to do with the color of water, but is simply the blue of the sky seen by reflection". Rayleigh had correctly described the nature of the blue sky by a phenomenon now known as Rayleigh scattering, the scattering of light and refraction by particles in the atmosphere. His explanation of the blue colour of water was instinctively accepted as correct. Raman could view the water using a Nicol prism to avoid the influence of sunlight reflected by the surface. He described how the sea appears even more blue than usual, contradicting Rayleigh.
As soon as the S.S. Narkunda docked in Bombay Harbour (now Mumbai Harbour), Raman finished an article "The colour of the sea" that was published in the November 1921 issue of Nature. He noted that Rayleigh's explanation is "questionable by a simple mode of observation" (using Nicol prism). As he thought:
When he reached Calcutta, he asked his student K. R. Ramanathan, who was from the University of Rangoon, to conduct further research at IACS. By early 1922, Raman came to a conclusion, as he reported in the Proceedings of the Royal Society of London:True to his words, Ramanathan published an elaborate experimental finding in 1923. His subsequent study of the Bay of Bengal in 1924 provided the full evidence. It is now known that the intrinsic color of water is mainly attributed to the selective absorption of longer wavelengths of light in the red and orange regions of the spectrum, owing to overtones of the infrared absorbing O-H (oxygen and hydrogen combined) stretching modes of water molecules.