Vacuum polarization in QED in the presence of some configurations of external magnetic fields is investigated. The configurations considered correspond to sourceless fields lying in a plane. The motion of a Dirac electron in these field configurations is studied and arguments are found to conclude that the lowest level gives the most important contribution. The result is that the main effect is not very different from the uniform case, the possibility of calculating the corrections due to the non-uniformity is explicitly shown. A typical effect of non-uniformity of the field is shown by the refractivity of the vacuum. .