  We have derived the differential equation governing the evolution of the
photon sphere for dynamical black hole spacetimes with or without spherical
symmetry. Numerical solution of the same depicting evolution of the photon
sphere has been presented for Vaidya, Reissner-Nordstr\"{o}m-Vaidya and
de-Sitter Vaidya spacetimes. It has been pointed out that evolution of the
photon sphere depends crucially on the validity of the null energy condition by
the in-falling matter and may present an observational window to even test it
through black hole shadow. We have also presented the evolution of the photon
sphere for slowly rotating Kerr-Vaidya spacetime and associated structure of
black hole shadow. Finally, the effective graviton metric for
Einstein-Gauss-Bonnet gravity has been presented, and the graviton sphere has
been contrasted with the photon sphere in this context.
