  The Casimir energy is computed in the geometry of interest for the most
precise experiments, a plane and a sphere in electromagnetic vacuum. The
scattering formula is developed on adapted plane-waves and multipole basis,
leading to an expression valid for arbitrary relative values of the sphere
radius and inter-plate distance. In the limiting case of perfect reflection,
the electromagnetic result is found to depart from the commonly used
proximity-force approximation (PFA) significantly more rapidly than expected
from scalar computations.
