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59
Text 9
THE COLOR OF THE SEA
The sea, the sky and other features of the natural world have provided inspiration for
many of the great scientific discoveries of humankind. The world of optics is no
exception. In 1921, C.V. Raman took an oceanic voyage that led him to study the
scattering of light and ultimately discover the vibrational effect that would come to
bear his name.
Recently I took a vacation on a cruise ship in the Sea of Cortez off Baja California.
Looking out at the fabulous blue color of the sea, I was put in mind of
Chandrasekhara Venkata (C.V.) Raman, a boy genius and Palit Chair of Physics at
Calcutta University by the age of 29. Raman made his first trip abroad by ship to
attend the Congress of the Universities of the British Empire in London in 1921. That
trip was to be of singular importance in Raman's life and the field of optics.
Prior to this journey, Raman had spent much of his time studying vibrational and
acoustical effects of various kinds. During his stay in London, he visited St. Paul's
Cathedral. He was fascinated by the "whispering gallery:" an acoustic phenomenon
that allowed a whisper uttered on one side of the circular gallery at the base of St.
Paul's great dome to be heard on the opposite side, some 43 m away. Before he left
London, Raman had published one of his many letters to Nature
explaining the phe-
nomenon and refuting an earlier theory put forward by Lord Rayleigh, 1904 Nobel
laureate in physics. This simple tourist encounter is at least partly responsible for the
whispering gallery mode lasers in use today ... but I digress.
Raman traveled to and from England by sea. It is said that he sat for hours on the
upper deck of the ocean liner staring at the deep blue color of the
Mediterranean. During the voyage, Raman conducted experiments peering into
the depths of the water using the Nicol (calcium calcite polarization) prism he
always carried. He sent a second letter to Nature, titled, "The Colour of the Sea,"
before even setting foot on dry land, from the .S. S. Narkunda) in Bombay
Harbour on September 26, 1921.
Rayleigh had proposed that the ocean's blue color resulted from reflected sky light
and absorption by matter suspended in the water. Raman's experiments clearly
showed that the blue color was independent of reflection and absorption and due
instead to molecular diffraction—most likely from water molecules themselves.
These simple experiments led Raman to pursue more detailed studies of light
scattering once back home in Calcutta.
In 1923, Compton proved the concept put forward by Planck and Einstein that
radiation is not only wave-like, but also particle-like, in nature. When a beam of
radiation traverses an atom, most of the radiation is elastically scattered and therefore
of unchanged wavelength. Using a graphite target, Compton showed that a small
fraction of the radiation emerged in directions other than that of the incident beam
due to inelastic scattering of the X-rays by electrons. Inspired by Compton's
discovery, Smekal predicted that photons should likewise be scattered inelastically by
vibrational transitions within molecules.
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