  This paper models light scattering through flat surfaces with finite
transmission, reflection and absorption rates, with wave packets approaching
the mirror from both sides. While using the same notion of photons as in free
space, our model also accounts for the presence of mirror images and the
possible exchange of energy between the electromagnetic field and the mirror
surface. To test our model, we derive the spontaneous decay rate and the level
shift of an atom in front of a semi-transparent mirror as a function of its
transmission and reflection rates. When considering limiting cases and using
standard approximations, our approach reproduces well-known results but it also
paves the way for the modelling of more complex scenarios.
