# Photon sphere and reentrant phase transition of charged Born-Infeld-AdS black holes

## TL;DR

This paper investigates how the photon sphere properties relate to different thermodynamic phase transitions, including reentrant transitions, in charged Born-Infeld-AdS black holes, revealing distinct behaviors and universal critical exponents.

## Contribution

It extends the analysis of photon sphere and phase transition relationships to Born-Infeld-AdS black holes, highlighting behaviors for various phase transition types and confirming universal critical exponents.

## Key findings

- Photon sphere radius and impact parameter vary with phase transition type.
- Reentrant phase transition exhibits unique temperature and pressure behaviors.
- Universal critical exponent of 12 near the critical point.

## Abstract

In this paper, we 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 decrease-increase-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 12. 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.