  Modified gravity theories are of great interest in both observational and
theoretical studies. In this article we study the correlation between the null
geodesics in the background of a four-dimensional Einstein-Maxwell-Gauss-Bonnet
AdS black hole, a modification of Einstein gravity, and it's thermodynamic
phase transition. We study the phase structure of the black hole, using the
coexistence and spinodal curves, to understand the phase transition in a
extended phase space. The imprints of this phase transition features are
observed in the behaviour of photon orbit radius and minimum impact parameter
with respect to the Hawking temperature and pressure. The change in these two
quantities during the phase transition serve as order parameter which
characterises the critical behaviour. The correlation shows that thermodynamic
phase transition can be studied by observing the effects of strong
gravitational field and vice versa.
