  In this work we establish some results concerning the existence of external
light rings in extremal black hole spacetimes through the Newman-Penrose
formalism. Specifically, assuming conformal flatness, staticity and the null
energy condition, we show that a sufficient condition for the existence of
external light rings is $R < 2 K_{G}$, where $R$, the curvature scalar of the
spacetime and $K_{G}$, the Gaussian curvature of a spacelike two-surface, are
both evaluated at the outermost event horizon, which can be endowed with
spherical, hyperbolic or planar geometry. Our results are valid for any metric
gravity theory where photons follow null geodesics.
