Whenever a charged particle is accelerated it radiates electro-magnetic waves. The only exception to this rule is when an electron is bound to a nucleus in an atom. There are certain states in which the electron can exist that are special and the electron will not radiate. These states are the allowed energy levels. When an electron changes an energy level, however, it will radiate. Consider a much simpler problem. Take a positively charged particle and accelerate it briefy allowing it to coast at constant velocity thereafter, so the particle has experienced an impulse. Two representations of the electric field are presented below. The first is the electric field line representation and the second is a magnitude plot of the electric field. Our goal is to understand what is happening in these movies.
Stop the film at frame 7. Notice that the strongest fields are radiated perpendicular to the direction of the acceleration. Why?
Go to Perpendicular.