The electromagnetic field is quantized on the basis of the classical spatial modes of a system geometry in which half of space is filled by a dielectric of constant refractive index and the other half of space is empty. The vacuum field fluctuations and spontaneous emission rates are evaluated as functions of position and polarization both inside and outside the dielectric. Particular attention is given to the variations of these quantities in the vicinity of the interface. The source-field operator is derived for emission by atoms inside and outside the dielectric, in the direction perpendicular to the interface.
