  We consider a planar optical cavity with its main axis oriented perpendicular
to the earth's local gravity field. We examine the motion of a wavepacket of
light trapped within the cavity spacer under the action of gravity. Within a
relativistic framework, we find that the wavepacket initially at rest in terms
of in-plane group velocity is accelerated by $g$ at early time in agreement the
non-relativistic limit of free-fall. We show that this phenomenon is a
manifestation of gravitational redshift, applied on photons at rest within the
cavity.
