In this paper, the shadows cast by non-rotating and rotating modified gravity black holes are investigated. In addition to the black hole spin parameter a and the inclination angle θ of observer, another parameter α measuring the deviation of gravitational constant from the Newton one is also found to affect the shape of the black hole shadow. The result shows that, for fixed values of a/M and θ, the size and perimeter of the shadows cast by the non-rotating and rotating black holes significantly increase with the parameter α, while the distortions decrease with α. Moreover, the energy emission rate of the black hole in high energy case is also investigated, and the result shows that the peak of the emission rate decreases with the parameter α.

Comparing to the charged AdS black hole in GR, a new interesting phase transition, the reentrant phase transition, is observed in a charged Born-Infeld-AdS black hole system. It is worth to extend the study of the relationship between the photon sphere and the thermodynamic phase transition, especially the reentrant phase transition, to this black hole background. According to the number of the thermodynamic critical points, the black hole systems are divided into four cases with different values of Born-Infeld parameter b, where the black hole systems can have no phase transition, reentrant phase transition, or Van der Waals-like phase transition. For these different cases, we obtain the corresponding phase structures in pressure-temperature diagram and temperature-specific volume diagram. The tiny differences between these cases are clearly displayed. On the other hand, the radius rps and the minimal impact parameter ups of the photon sphere are calculated via the effective potential of the radial motion of photons. For different cases, rps and ups are found to have different behaviors. In particular, with the increase of rps or ups, the temperature possesses a decreaseincrease-decrease-increase behavior for fixed pressure if there exists the reentrant phase transition.While for fixed temperature, the pressure will show an increase-decrease-increase-decrease behavior instead. These behaviors are quite different from that of the Van der Waals-like phase transition.Near the critical point, the changes of rps and ups among the black hole phase transition confirm an universal critical exponent 1 2 . Finally, we also find that the temperature and pressure corresponding to the extremal points of rps and ups are highly consistent with the thermodynamic metastable curve for the black hole systems with different values of b. Therefore, all the results indicate that, for the charged Born-Infeld-AdS black holes, not only the Van der Waals-like phase transition, but also the reentrant phase transition can be reflected through the photon sphere.