Curvatures, Photon Spheres and Black Hole Shadows

In a recent work PRD 106 L021501, a new geometric approach is proposed to obtain the photon sphere (circular photon orbit) and black hole shadow radius. In this approach, photon spheres and black hole shadow radius are determined using geodesic curvature and Gaussian curvature in the optical geometry of black hole spacetimes. However, the calculations in PRD 106 L021501 only restricted to a subclass of static and spherically symmetric black holes with spacetime metric $g_{tt} \cdot g_{rr}=-1$ and $g_{\theta\theta}=r^{2}$. In this work, we extend this approach to more general spherically symmetric black holes (with spacetime metric $ds^{2}=g_{tt}dt^{2}+g_{rr}dr^{2}+g_{\theta\theta}d\Omega^{2}$). Furthermore, it can be proved that our results from the geometric approach are completely equivalent to those from conventional approach based on effective potentials of test particles.Comment: 12 pages, 3 figures, 3 tables. arXiv admin note: text overlap with arXiv:2204.0729