Cherenkov radiation (/tʃəˈrɛŋkɒf/; Russian: Черенков) is electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium at a speed greater than the phase velocity (speed of propagation of a wave in a medium) of light in that medium. Special relativity is not violated since light travels slower in materials with refractive index greater than one, and it is the speed of light in a vacuum which cannot be reached (or exceeded) by particles with mass. A classic example of Cherenkov radiation is the characteristic blue glow of an underwater nuclear reactor. Its cause is similar to the cause of a sonic boom, the sharp sound heard when faster-than-sound movement occurs. The phenomenon is named for Soviet physicist Pavel Cherenkov, who shared the 1958 Nobel Prize in Physics for its discovery.