Ralph Kronig (born 10.03.1904) is a Dutch theoretical physicist and a member of the Royal Netherlands Academy of Arts and Sciences. He was born in Dresden. He graduated from Columbia University. In 1931 —39 he was a lecturer at the University of Groningen, and in 1939 —69 professor and in 1959 —62 rector of the Higher Technical School in Delft.
His work was in the fields of spectroscopy, the theory of molecular structures, the theory of valence, quantum mechanics, nuclear physics and solid-state physics. Independently of S. Goudsmit and G. Uhlenbeck he arrived (early 1925) at the concept of electron spin. In 1926, independently of H. Kramers, he obtained the dispersion relations in classical electrodynamics (the Kramers–Kronig relations). He gave a qualitative picture of the behaviour of electrons in a crystal made up of a sequence of identical potential barriers and rectangular potential wells (the Kronig–Penney model). In 1939, independently of J. Van Vleck, he proposed a mechanism of spin-lattice relaxation (the Kronig–Van Vleck mechanism).
The M. Planck Medal (1962).