The IISc Titan Who Discovered the "Krishnan Reciprocity Effect" If one walked through 18th Cross in Sadashivanagar, right on the edge of Malleswaram, Bangalore, in the mid to late 20th century, s/he would frequently see an elderly gentleman with a traditional South Indian demeanor walking towards the Physics Department at the IISc. To his neighbors, he was simply an elderly retired prof who lived a quiet life. Hardly anyone in the neighborhood realized that this man was C. V. Raman’s former student and successor, a nuclear physics researcher at Cambridge's Cavendish Laboratory, and the discoverer of an optical symmetry relation that bears his name. Born in September 1911 in Palakkad, Kerala, Rappal Sangameswaran Krishnan moved to Bangalore in 1933 to join IISc under Nobel laureate Sir C. V. Raman after graduating from St. Joseph’s College, Tiruchirappalli. While experimenting on the polarization of light scattered by colloid particles and liquid mixtures, the young researcher observed a universal symmetry relation: V_h = H_v (where the intensity component of horizontally polarized scattered light from a vertically polarized incident beam equals that of a vertically polarized scattered light from a horizontally polarized beam). This relationship, published between 1934 and 1938, became known in optical physics as the "Krishnan Reciprocity Effect". It provided a vital experimental and theoretical framework for analyzing light scattering in colloids, polymers, and liquid crystals. Recognizing his experimental talent, Raman supported Krishnan's application for an 1851 Exhibition Scholarship. In 1938, Krishnan arrived at the world-famous Cavendish Laboratory at the University of Cambridge. Krishnan joined the laboratory under John Cockcroft and completed his doctoral thesis under Norman Feather: He was put in charge of running and maintaining the lab’s 37 inch cyclotron. Using high-energy deuterons from the cyclotron, he was reported as the 1st to observe deuteron-induced fission in heavy elements like uranium and thorium. He earned his Ph.D. from Cambridge in 1941 before returning to India during the height of World War II. Upon returning to Bangalore, Krishnan re-joined IISc. In 1948, when C. V. Raman retired from IISc to set up the Raman Research Institute, Krishnan succeeded his mentor as the Head of the Department of Physics at IISc, a post he held for a quarter of a century (1948-73). While C. V. Raman was the famous face of Indian science, Krishnan built up IISc's experimental infrastructure: - He pioneered experimental studies of Raman and Brillouin spectra in solids, particularly diamonds, quartz, and alkali halides using the 2537 Å ultraviolet line of mercury from custom fused-quartz lamps built in-house. - At a time when forex crunches made importing scientific hardware difficult, Krishnan fostered a strong workshop culture to fabricate experimental setups locally. - He personally mentored 60 Ph.D. students and published 220+ research papers. He later served as the Vice-Chancellor of Kerala University (1973-77) and spent his late years compiling comprehensive sourcebooks on the Raman effect. Throughout his decades at IISc and after his retirement, Professor R. S. Krishnan lived on 18th Cross Road in Sadashivanagar, just a short walk from both the IISc campus gates and Raman's RRI. Unlike the flamboyant Raman, Krishnan shunned personal publicity. He spent his final years walking to academic libraries and compiling historical physics records. When he passed away in Beangalore in October 1999 at the age of 88, he left behind a lasting legacy in crystal physics and optics, a giant within Indian physics who lived with virtually no public fame outside academic walls. P.S. Do not confuse him with K. S. Krishnan (1898-1961). That Krishnan was C. V. Raman’s collaborator in Calcutta on the 1928 discovery of the Raman effect. R. S. Krishnan was a later Raman student at IISc, known for the Krishnan reciprocity effect in colloid scattering and for heading IISc Physics after Raman. Same surname, same orbit, two different scientists. 🙏