  The interaction of the electromagnetic field with a two dimensional plasma
sheet intended to describe the pi-electrons of a carbon nano-tube or a $C_{60}$
molecule is investigated. By integrating out first the displacement field of
the plasma or first the electromagnetic field different representations for
quantities like the Casimir energy are derived which are shown to be consistent
with one another. Starting from the covariant gauge for the electromagnetic
field it is shown that the matching conditions to which the presence of the
plasma sheet can be reduced are different from the commonly used ones. The
difference in the treatments does not show up in the Casimir force between two
parallel sheets, but it is present in the Casimir-Polder force between a charge
or a neutral atom and a sheet. At once, since the plasma sheet is a
regularization of the conductor boundary conditions, this sheds light on the
difference in physics found earlier in the realization of conductor boundary
conditions as 'thin' or 'thick' boundary conditions in
Phys.Rev.D70(2004)085010.
