If we drop a pebble into a pond, a circular ripple will spread out on the surface of water. If we continue dropping pebbles at regular intervals then a series of concentric rings all moving outwards with the same speed will be formed. However what happens if we drop pebbles by moving our hand linearly in one direction or horizontal direction? A series of circular waves will be produced that are no longer concentric however rather crowded in the direction where hand is moving. If we move the hand with the speed with which the rings move then all the rings will touch at a point. What happens if we move the source of the ripples faster than the speed of rings with which they spread. . Cherenkov radiation is simply a type of electromagnetic radiation which is observed when a charged particle moves with a faster speed than the speed of light in that medium in which charged particle is moving. Speed of the particle should be faster than the phase velocity of light instead of overall velocity .