  An exact formulation of the propagation of a monochromatic wave packet
impinging upon a transparent, homogeneous, isotropic and parallel slab at
oblique incidence is presented. Approximate formulas are derived for low
divergence Gaussian light beams. These formulas show the presence of anomalous
refraction phenomena at any slab thickness, including negative refraction and
flat lensing effects, induced by the reflection on the rear face.
