Modification of spontaneous emission rate of an atom in the center of a hemispherical mirror. Spontaneous emission of an atom at the center of curvature of a spherical mirror is inhibited or enhanced depending on the radius and numerical aperture of the mirror. If the radius of the mirror is R = nλ/2, the center of curvature is a node of the vacuum mode density and inhibition is expected (blue curve). If the radius of the mirror is R = nλ/2 + λ/4, the center of curvature is an anti-node of the vacuum mode density and enhancement is expected (orange curve). Full inhibition (respectively, a doubling of the decay rate) is obtained when half of the space is covered by the mirror (NA = 1). In this plot, a mirror with perfect reflectivity is assumed.