Rappal Sangameswaraier Krishnan (1911–1999) was an Indian scientist and researcher.
Born in 1911 at Rappal,Thrissur District. Dr.R.S.Krishnan was one of the greatest Indian scientists. After obtaining M.A. and D.Sc. from the Madras University, he joined as a researcher at the Cavendish Laboratory of the Cambridge University. He played a vital role in developing 37’ Cyclotron. Later he held full charge of the instrument and its maintenance. There he observed the deuteron-induced fission in uranium and thorium. In 1941 he was awarded Ph.D.Degree by the Cambridge University. On his return to India, he was appointed in the Physics Department of Indian Institute of Sciences, Bangalore and worked under the famous scientist Dr.C.V.Raman. He is well known for the new effects that he discovered in colloid optics. He discovered the reciprocity relations between the intensity of the horizontally polarised incident light getting scattered with horizontal polarization irrespective of the colloidal particles. This is known as Krishnan (reciprocity) Effect. His work on the Second Order Raman Spectra in diamond and in alkali halide crystals has also gained worldwide recognition. In 1948 when Dr.C.V. retired, Dr.Krishnan became the Head of Department of Physics of Indian Institute of Science, Bangalore. He represented India in several International Conferences and Seminars like 2nd International Conference on crystallography, Stockholm (1951), International Science Conference at Edinburough, 5th Australian Spectroscopic Conference, 1st International Conference on ‘Raman Spectra on Crystals’ at Paris (1965) etc. He guided more than 60 students in their Ph.D.programme and published more than 500 research articles in various international journals. He was a member of Indian Academy of Sciences, London Institute of Science, American Physical Society. He was a visiting Professor to many universities of Europe and United States of America. After his retirement from the Indian Institute of Sciences, Bangalore in 1972, Dr.Krishnan was appointed as the Vice-Chancellor of the University of Kerala in 1973 and continued in his office till 1977. After his retirement he returned to Bangalore and spent the remaining period of his life for compiling the research publications related with Raman Effect and authoring some science works.His main source of support who stood by him was his wife Mrs.Narayani Krishnan, after whom his youngest grand daughter is named. He died on October 2, 1999. Also it is understood that Mr.Krishnan is the father-in-law of Mr.T.N.Seshan (Former Chief Election Commissioner, India).