where [ P ( r ′ ) ] and [ M ( r ′ ) ] should be evaluated at the retarded time | r − − r ′ | / v . The electric and magnetic fields can then be found using the Hertz vectors. For simplicity in observing the relationship between polarization, the Hertz vectors, and the fields, only one source of polarization (electric or magnetic) will be considered at a time. In the absence of any magnetic polarization, the Π Π e vector is used to find the fields as follows: