Medium-induced optical effects for prompt photons

Electromagnetic aspects of a QCD matter have been a hot topic in recent years. High-energy heavy-ion experiments revealed that flow harmonics of direct photons are not explained by most hydrodynamic models. In this work I discuss possible effects of refraction by the hot medium since it can work as a lens which converts its geometrical anisotropy into momentum anisotropy of photons. Then elliptic flow and higher-order harmonics of prompt photons are estimated numerically. The results indicate that they could be small but have phenomenologically non-trivial consequences.