  We probe the phase structure of the regular AdS black holes using the null
geodesics. The radius of photon orbit and minimum impact parameter shows a
non-monotonous behaviour below the critical values of the temperature and the
pressure, corresponding to the phase transition in extended phase space. The
respective differences of the radius of unstable circular orbit and the minimum
impact parameter can be seen as the order parameter for the small-large black
hole phase transition, with a critical exponent $1/2$. Our study shows that
there exists a close relationship between the gravity and thermodynamics for
the regular AdS black holes.
