Why is the sea blue?

It is well known in the history of science that study of some natural phenomenon has been a starting-point in the development of a new branch of knowledge. One such example is the following. In 1919, Sir C.V. Raman was studying the phenomenon of diffraction and molecular light scattering, especially in the context of liquids. His interest in this topic was aroused by an interesting experiment: a beam of white light was passed through a tank containing a solution. Certain chemicals were then added to this solution so that it gradually changed from a clear liquid into a turbid one. This was due to the production of particles which then remained suspended in the liquid. Naturally, the intensity of the transmitted light decreased as the turbidity increased, and at one stage the light was almost cut off. Interestingly, it was found that with the further passage of time, not only the intensity of the transmitted light increased, but also its colour went through a series of changes — indigo, blue, blue-green, greenish-yellow, and finally white. Why did such a thing happen? An explanation was necessary. Lord Rayleigh was able to explain the initial decrease in intensity of the transmitted light but had no answer for the strange appearance of colours later. Raman was able to explain this. First, he considered the way the light wave is diffracted by the individual particles, and then how these different diffracted waves combined together, either constructively or destructively, to produce an overall effect. There was one more intriguing question: What if the diffraction is not by a suspended particle but by a molecule? In 1921, Raman was returning to India via sea after his first ever visit abroad. He was fascinated by the deep blue colour of the Mediterranean and began to wonder why the sea is blue. Earlier, Lord Rayleigh who had successfully explained the blue colour of the sky had declared, "The much-admired dark blue of the deep sea has nothing to do with the colour of water but is simply the blue of the sky seen in reflection." In short, the sea is blue because it is merely reflecting the blue sky — this was Lord Rayleigh's explanation. Raman was not satisfied with this explanation, and further investigated this matter in detail . His observations revealed the following: (i) light can be scattered by the molecules of water just as it can be by the molecules of air, and (ii) that the blue colour of the sea is due to such molecular scattering just as the blue of the sky is. In his seminal paper on the molecular scattering of light, Raman concluded the following: "In this phenomenon, as in the parallel case of the colour of the sky, molecular diffraction determines the observed luminosity and in great measure also its colour." Raman thus proved that the sea is blue because the molecules of water scatter light just the same way molecules of air do. This was an important result which not only disproved Lord Rayleigh's explanation, but also had further implications in understanding the interaction of light with molecules, especially in the context of Raman effect.