Cartoon of expanding dipole wavefronts 

This animation is a cartoon of a free electron forced into oscillations by a vertically polarized sinusoidal planar electric wave. The electron moves exactly 180o out of phase with the driving E-field amplitude. Due to its sinusoidal acceleration, the electron emits Hertzian dipole radiation with the nodal axis defining the axis of acceleration. In this example, this axis is vertical. The cartoon shows four expanding wavefronts at multiples of the driving periodicity.