In 1934, Frank moved to the Institute of Physics and Mathematics of the USSR Academy of Sciences (which shortly would be moved to Moscow, where it was transformed into the Institute of Physics). Here he started working on nuclear physics, a new field for him. He became interested in the effect discovered by Pavel Cherenkov, that charged particles moving through water at high speeds emit light. Together with Igor Tamm, he developed a theoretical explanation: the effect occurs when charged particles travel through an optically transparent medium at speeds greater than the speed of light in that medium, causing a shock wave in the electromagnetic field. The amount of energy radiated in this process is given by the Frank–Tamm formula.