The leading semiclassical estimates of the electromagnetic Casimir stresseson a spherical and a cylindrical metallic shell are within 1% of the fieldtheoretical values. The electromagnetic Casimir energy for both geometries isgiven by two decoupled massless scalars that satisfy conformally covariantboundary conditions. Surface contributions vanish for smooth metallicboundaries and the finite electromagnetic Casimir energy in leadingsemiclassical approximation is due to quadratic fluctuations about periodicrays in the interior of the cavity only. Semiclassically the non-vanishingCasimir energy of a metallic cylindrical shell is almost entirely due toFresnel diffraction.
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