Ilya Frank worked on the experiments and theoretical interpretation of Cherenkov radiation. With Igor Tamm, he explained in 1937 why radiation from a sufficiently fast charged particle adds coherently in a medium. The emission direction is linked to the particle's speed and the refractive index, so an observed optical pattern can carry particle-identification information. His later work also established a substantial programme in neutron physics.[ref]
Ilya Mikhailovich Frank: first FLNP JINR director, laureate of Nobel Prize by Joint Institute for Nuclear Research. Participation in the liquid-irradiation work, 1937 theory and later neutron-physics programme
A physicist.[ref]
https://www.nobelprize.org/prizes/physics/1958/frank/biographical/. Independent professional profile
Portrait of Ilya Frank
. Nobel Foundation. Public domain in Sweden per Commons PD-Sweden photo tag; separate U.S. status tag not present on inspected File. Source usage terms, via Wikimedia Commons. Source.
