Universal Boundary Relations of the Electromagnetic Field

In the present paper it is shown that a set of universal boundary relations which give the discontinuities of the electromagnetic field across any regular surface can be written via the distribution concept. All specific boundary conditions are then derived from these universal relations as particular cases. It is also shown that any set of linear relations written on a regular surface can not always be the boundary relations of an electromagnetic field. In the case of monochromatic fields the necessary and sufficient conditions for a set of linear equations to be the boundary conditions are given in three theorems. These theorems are applied to the discussion of the validity of boundary conditions of impedance type and show that such conditions can only be valid at certain critical frequencies.