  We study null bulk geodesic motion in the brane world cosmology in the RS2
scenario and in the static universe in the bulk of the charged topological AdS
black hole. We obtain equations describing the null bulk geodesic motion as
observed in one lower dimensions. We find that the null geodesic motion in the
bulk of the brane world cosmology in the RS2 scenario is observed to be under
the additional influence of extra non-gravitational force by the observer on
the three-brane, if the brane universe does not possess the Z_2 symmetry. As
for the null geodesic motion in the static universe in the bulk of the charged
AdS black hole, the extra force is realized even when the brane universe has
the Z_2 symmetry.
