P is a fixed point at a distance l from the equilibrium position of the particle. Because l > a, to first order in a/l the distance between P and the instantaneous position of the particle is still l. For the first case y = 0 so t = T/4
The corresponding retarded time is
For the second case y = a at t = 0 so at the retarded time
The radiation fluxes in the two cases are proportional to
Note : The radiation received at P at time t depends on the acceleration of the charge at the retarded time.