Raman, Chandrasekhara Venkata
RAMAN, CHANDRASEKHARA VENKATA
(b. Tiruchirapalli [Trichinopoly], India, 7 November 1888; d Bangalore, India, 21 November 1970)
physics, physical theory, physiology of vision.
Raman was the son of Chandrasekhara Aiyar, professor of mathematics and physics at the A.V.N. College in Vizagapatam, and Parvati Ammal, who belonged to a family known for Sanskrit scholarship. From his father Raman gained an early interest in science and a love of music and musical instruments; his mother contributed to his sense of self-reliance and his strong personality. Raman was educated at the A.V.N. College and at the Presidency College of the University of Madras, from which he received the B.A. in 1904, when he was sixteen. He ranked first among the students and was awarded the gold medal for physics. He then read for the M.A. degree, which he was granted with highest honors in January 1907; during this time he also, in 1906, published his first paper, on the unsymmetrical diffraction bands produced by a rectangular aperture. Among the works that he studies, Rayleigh’s Theory of Sound and Helmholtz’ Sensations of Tones influenced him profoundly.
III health prevented Raman from pursuing higher studies in physics at one of the great British universities and India offered him no possibility of a further career in science. He therefore decided to enter the civil service. The Indian audit and accounts service was at this time the only department that did not require a training period in Great Britain, and Raman took the examination for a position in the Indian finance department. He placed first, and in June 1907 was posted to Calcutta as assistant accountant general. In the same year he married Lokasundari, an accomplished artist who shared his interest in musical instruments. Raman served in the finance department for ten years, in posts of increasing responsibility. At the same time he continued to perform research, particularly on vibrations and sound (including experimental and theoretical studies of the oscillations of strings) and on the theory of musical instruments, particularly the violin family and Indian drums. Most of this work was accomplished at the laboratory of the Indian Association for the Cultivation of Science, which had been founded in Calcutta by Mahendralal Sircar in 1876.
Raman’s independent research, and the many publications that resulted from it, led to his being invited, in 1917, to fill the newly established Palit professorship of physics at the University of Calcutta. He gave up his lucrative government career to accept it. He held the chair for sixteen years, during which he continued to do research on acoustics and optics at the Association. He was assisted in his efforts by a growing number of collaborators and visitors; during this period such men as Meghnad Saha and S. N. Bose, inspired by Raman’s example and success, rose to prominence as Calcutta became a center for scientific research. In the summer of 1921 Raman represented the University of Calcutta at the British Empire Universities Congress held at Oxford. While in England he also lectured on the theory of stringed instruments before the Royal Society.
Raman returned to India by way of a voyage through the Mediterranean. He was struck by the deep opalescent blue of the sea, and, back in Calcutta, undertook to discover its cause. Rayleigh had explained the azure of the sky as being caused by the scattering of sunlight by molecules in the gaseous atmosphere, but had gone on to state that the color of the sea had nothing to do with the color of the water, but rather merely reflected the blue of the sky. He had then further speculated that the apparent blueness of the sea might be caused by the absorption of light by the water, since “if a liquid is not absolutely clear, but contains in suspension very minute particles, it will disperse light of a blue character.”
Raman did not find Rayleigh’s explanation convincing, and in a paper published in the Proceedings of the Royal Society in 1922 he demonstrated that the scattering of light by water molecules could account for the color of the sea in precisely the same manner that the scattering of light by air molecules could explain the color of the sky. Raman applied the Einstein-Smoluchowski theory of fluctuations in his study and further obtained experimental confirmation that water under normal conditions scatters light into an angle of 30°—about 150 times greater than scattering of sunlight by air free of dust particles. His co-workers K. Seshagin and K. S. Krishnan then tested and generalized the Einstein-Smoluchowski theory itself.
Although he had begun to concentrate upon optics, Raman continued his work in acoustics as well. His researches in this field were admired by Rayleigh, and it was largely because of them that Raman was elected a fellow of the Royal Society in 1924. His definitive article, “Musikinstrumente und ihre Klänge,” appeared in the third volume of H. Geiger and K. Scheel’s Handbuch der Physik in 1927. Only the linear theory of vibrations of ideal strings had been developed before Raman began his investigations, and he obtained new results on the excitation of string vibrations, the motion of the bowed point, and on the effect of the bridge in coupling the motion of the string to the body of the violin. He also made quantitative studies of the vibration phenomena of the piano, the sitar, and the veena, and showed that the mridangam and the tabla—Indian musical drums—possess harmonic overtones specific to their drumheads that do not normally occur in circular membranes.
Raman also traveled extensively during this time. In 1924 he spoke on the scattering of light before the meeting of the British Association for the Advancement of Science, held in Toronto in 1924, then journeyed across Canada and the United States. He represented India at the centennial celebrations of the Franklin Institute in Philadelphia, then, at the invitation of Robert A. Millikan, spent five months at the California Institute of Technology in Pasadena. Following his return to India early in 1925 Raman went to the U.S.S.R. to attend the bicentennial ceremonies of the Academy of Science; on his return trip he visited scientific institutions in Germany, Switzerland, and Italy. He was also active in organizing learned institutions in his own country; he was among the founders of the Indian Science Congress in 1924, and served as its secretary for several years, later becoming president of its Madras convention. In 1926 he established the Indian Journal of Physics.
Optical studies remained his chief concern, however. With his associates Raman studied the scattering of light of available frequencies by a number of substances, particularly fluids. In April 1923 Raman’s associate K. R. Ramanathan observed a weak secondary radiation, shifted in wavelength along with normally scattered light, which was attributed to “fluorescence.” S. Venkateswaran then noticed that highly purified glycerin does not appear blue under sunlight, but rather radiates a strongly polarized, brilliant green light.
Raman and K. S. Krishnan then undertook to isolate the effect under impeccable experimental conditions. They employed complementary light filters placed in the paths of the incident and scattered light, respectively, and observed a “new type of secondary radiation” from the scattering of focused beams of sunlight in both carefully purified liquid and dust-free air. They reported this discovery in a letter to Nature in February 1928. Raman then refined the experiment by using a mercury arc as the source of light; the effect was thus clearly seen for the first time on 28 February 1928 and was reported to the Science Congress at Bangalore the following month. The secondary radiation showed several lines shifted toward longer wavelengths, the shifts being characteristic of the substances being examined, and indicated the absorption of energy by the scattering molecule—the precise effect that had been predicted by A. Smekal in 1923. G. Landsberg and L. Mandelshtam, in the U.S.S.R., independently observed the same phenomenon in quartz, shortly after Raman and Krishnan made their discovery, but Raman’s account of the effect reflected a much more detailed investigation. In 1929 Raman was knighted in recognition of his work, and the following year he was awarded the Nobel Prize for physics.
The changes of the wavelength of incident light in the Raman effect are caused by the internal motions of molecules of the scattering substance. Additional lines that are not present in the spectrum of the incident beam arc visible in the spectrum of the scattered light. While the shifts of wavelengths attributable merely to translational molecular movements are usually small, irregular, and unobservable, the rotation of molecules in gases gives rise to closely spaced Raman lines on either side of the incident line. In the case of a dense fluid, the collision of molecules hinders molecular rotation, so that the Raman lines develop as a continuous band. Since the internal vibrations of molecules lead to shifts of great wavelength, the Raman lines attributed to them appear well separated from the parent line.
The quantum theory offers a satisfactory explanation for the Raman effect by which it may be seen to arise from the exchange of energy between the light quanta and the molecules of the substance on which they impinge. The incident photon of energy, hv0is absorbed by the molecule, which is then translated into the intermediate state m, from which it returns to the final state f, emitting a photon in the process, so that hv0 — hvm = hvf. If the scattering molecule imparts energy hvm to the photon, the total energy of the photon becomes hv0 + hvm, which appears as scattered radiation of increased frequency v0 + vm. Energy is conserved only between the initial and final stages of this system, so that virtual transitions to a whole gamut of intermediate states are possible. The intensity ratio of lines with frequencies v0 + vm and v0 - vm may be expressed as a function of temperature, T, and characteristic frequency of the molecule vm as e-hvm/kT. R. W. Wood called the Raman effect “one of the most convincing proofs of the quantum theory of light.”
In 1933 Raman left Calcutta and went to Bangalore, where he served the Indian Institute of Science as both its president (1933–1937) and head of the physics department (1933–1948). In 1934 he founded the Indian Academy of Science, whose Proceedings have appeared continuously and regularly since that date. During this period Raman also continued his investigations of the Raman effect (as did others—almost 2,000 papers on the subject were published by other workers in the twelve years following its discovery). In 1935 and 1936, with N. S. Nagendra Nath,Raman published two important papers in the Proceedings of the Indian Academy of Science on the scattering of radiation by ultrasonic waves in a liquid. Earlier workers, including P. Debye and F. W. Sears in the United States and R. Lucas and P. Biguard in France, had utilized L. Brilliouin’s theory whereby the sound waves create a grating because of the rarefaction and condensation of the fluid. Raman and Nath, however,as a conclusion of their researches proposed instead a new theory of pure phase grating that could also account for the intensities of higher-order Bragg reflections.
In 1940 Raman began to look for a new approach to the dynamics of crystal lattices. In a series of articles published in 1941 and 1942, he assumed a crystal to be a geometrical array with a coherent structure of similarly placed atoms (and molecules and ions) capable of exerting mutual influence; he was thereby able to explain a finite number of discrete monochromatic frequencies that he had observed in the Raman spectra. In this he drew upon Rayleigh’s definition of a normal mode as being one in which all particles in a normal vibration oscillate at any instant with equal amplitude in the same or the opposite phase. Thus all particles in a crystal unit cell may be replaced by equivalent particles in adjacent cells, and the vibrations proceed along Bravais axes; the modes in a crystal therefore reduce to the modes in a supercell of twice the linear dimensions of a unit cell. The theory does not account for the continuum that is observed to be superimposed on the line spectra, a consideration that stimulated M. Born and his collaborators to reexamine the Born-Kármán theory of 1912, which was subsequently experimentally proved to be accurate.
In 1943 the government of Mysore gave the Indian Academy of Sciences eleven acres of land in Hebbal, a suburb of Bangalore. The Raman Research Institute was built on this site and, upon its completion in 1948, Raman became its first director. The government of the newly independent India appointed him national professor in the same year. He continued his research on optics and crystal structure—to which he added investigations of the physiology of vision—and his training of graduate students, many of whom he sent out into important positions throughout India.
From 1950 until 1958 Raman investigated optical effects in gems and minerals. He was particularly fascinated by diamonds—indeed, he had in the early 1930’s studied their Raman spectra, and in 1944 and 1946 he had participated in two symposia on the structure of diamonds, held at Bangalore, in which he and his co-workers reported on the fluorescence, luminescence, absorption spectra, magnetic susceptibility, and second-order Raman effects of that gem. In the 1950’s Raman returned to the subject and examined the specific heat, X-ray diffraction, and infrared spectrum of diamonds; in addition, he devised a new experimental technique in which he used a pencil beam of sunlight to demonstrate the reflections from gems and minerals as diffusion halos against a white screen. In particular, he studied iridescent feldspars, such as labradorite, which changes color with the angle of observation from peacock blue to green to golden yellow. He interpreted this effect as being caused by the mixture of chemical components within the lamellar structure of the mineral. He added that the Schiller effect of the moonstone is similarly caused, both of its components being birefringent. He also examined opals and pearls.
In the 1960’s Raman turned to investigating colors and their perception. He was perhaps the first to study flowers spectroscopically, and in a series of papers published in 1963 he set out the results of his researches on the colors of the petals and the spectra of various floral species, including the rose, aster, and hibiscus. In 1964 he began to publish a series of papers in which he attempted to establish a new theory of color vision, in opposition to the trichromacy theory of human color perception; in particular he rejected Maxwell’s work on the subject. These papers-there were eventually forty-three of them-were collected as The Physiology of Vision, published in 1968 in honor of Raman’s eightieth birthday.
Raman was a man of great personal authority and presence; he was proud, even arrogant, and could be contemptuous of the power and authority of others. He was a great teacher-he once stated that “the principal function of the older generation of scientific men is to discover talent and genius in the younger generation and to provide ample opportunities for its expression and expansion”—and was generous in encouraging his students. He delighted in the public exposition of science, and gave annual lectures on general scientific topics at the Raman Research Institute on 2 October, Gandhi’s birthday. He gave his last Gandhi Memorial Lecture, on the “Theory of Hearing,” a few weeks before his death. As a scientist, Raman was a poineer. Educated entirely in India, he did outstanding work at a time when the small Indian scientific community worked almost entirely in isolation and few made science a career; in fostering Indian science, Raman emerged as one of the heroes of the Indian political and cultural renaissance, along with Tagore, Gandhi, and Nehru.
Raman’s private universe was one of light and color, of sound and tonality. The colors of flowers and of precious stones and the sound of strings and drums were for him the elements of eternal experience, which he could order with the aid of science. He loved roses more than anything else, and maintained a large rose garden. He was cremated there following his death at the age of eighty-two.
Raman published more than 500 articles in philosophical Magazine, Nature, Physical Review, Proceedings of the Royal Society, Astrophysical Journal, Journal of the Optical Society of America, Zeitschrift für Physik, Bulletin. Indian Association for the Cultivation of Science, Proceedings of the Indian Association for the Cultivation of Science, and Indian Journal of Physics; from 1934 on his papers appeared almost exclusively in proceedings of the Indian Academy of Science and Current Science. Poggendorff gives an extensive bibliography of individual items.
Among Raman’s most important works are “Dynamical Theory of the Motion of Bowed Strings,” in Bulletin. Indian Association for the Cultivation of Science, 11 (1914); “On the Mechanical Theory of the Vibration of Bowed Strings and of Musical Instruments of the Violin Family with Experimental Verification of the Results,” ibid., 15 (1918); “On the Molecular Scattering of Light in Water and the Colour of the Sea,” in Proceedings of the Royal Society, 64 (1922); “Musikinstrumente und ihre Klängd,” in H. Geiger and K. Scheel, eds., Handbuch der physik, VIII (Berlin, 1927); “A New Type of Secondary Radiation,” in Nature, 121 (1928), 501, written with K. S. Krishnan; “A New Radiaton,” in Indian Journal of Physics, 2 (1928), 387; “The Diffraction of Light by High Frequency Sound Waves,” in Proceedings of the Indian Academy of Science, 2A (1935), 406, and 413, 3A (1936), 35, 75, 119, 495, written with N. S. Nagendra Nath; “Crystals and Photons,” ibid., 13A (1941), 1; “The Thermal Energy of Crystalline Solids; Basic Theory,” ibid., 14A (1941), 459; “New Concepts of the Solid State,” ibid., 15A (1942), 65; “Floral Colours,” ibid., 58A (1963), 57ff.; and “The New Physiology of Vision,” ibid., 60A (1964), 61A (1965), 62A (1965), 63A (1966), collected as The Physiology of Vision(Bangalore, 1968).