  A set of four scalar conditions involving normal components of the fields D
and B and their normal derivatives at a planar surface is introduced, among
which different pairs can be chosen to represent possible boundary conditions
for the electromagnetic fields. Four such pairs turn out to yield meaningful
boundary conditions and their responses for an incident plane wave at a planar
boundary are studied. The theory is subsequently generalized to more general
boundary surfaces defined by a coordinate function. It is found that two of the
pairs correspond to the PEC and PMC conditions while the other two correspond
to a mixture of PEC and PMC conditions for fields polarized TE or TM with
respect to the coordinate defining the surface.
