  We study the relationship between the null geodesics and thermodynamic phase
transition for the charged AdS black hole. In the reduced parameter space, we
find that there exist non-monotonic behaviors of the photon sphere radius and
the minimum impact parameter for the pressure below its critical value. The
study also shows that the changes of the photon sphere radius and the minimum
impact parameter can serve as order parameters for the small-large black hole
phase transition. In particular, these changes have an universal exponent of
$\frac{1}{2}$ near the critical point for any dimension $d$ of spacetime. These
results imply that there may exist universal critical behavior of gravity near
the thermodynamic critical point of the black hole system.
