  We prove that, under the condition of validity of the Fresnel approximation,
diffraction and interference for a wave traveling in the z-direction may be
described in terms of the spreading in time of the transverse (x,y)-wave
packet. The time required for the evolved wave packet to yield identical
patterns as given by standard optics corresponds to the time for the quantum to
cross the optical apparatus. This point of view may provide interesting cues in
wave mechanics and quantum physics education.
