Abstract
The propagation of a Gaussian-shaped beam through a slab of optically denser dielectric medium in the air, a two-dimensional optical counterpart of the quantum potential well, is investigated in order to elucidate the negative lateral shifts of transmitted and reflected beams. It is shown that the locus of the peak of the beam inside the slab consists of two parts. One propagates backwards and seems to be the geometric reflection of the other that propagates forwards. But they are not continuous with the loci of the peaks of the incident and reflected beams. A restriction to the thickness of the slab is obtained that is necessary for the shape of the beam to remain unchanged in the travelling.