Rappal Sangameswaran Krishnan (born 23.09.1911) is an Indian physicist and a member of the Indian Academy of Sciences. He was born in Rappal. He graduated from the University of Madras (1937), and in 1941 he received the degree of Doctor of Philosophy at the University of Cambridge. In 1942 — 72 he worked at the Indian Institute of Science in Bangalore (from 1958 as professor and head of the physics department, and from 1970 as professor), and in 1973 — 77 he was vice-chancellor of the University of Kerala.
His research is in the fields of molecular optics, the physics of crystals and nuclear physics. In colloid optics he is known for the “Krishnan effect” in liquids.