  For a model of the electromagnetic field coupled to Weyl tensor, Maxwell
equations are modified and photons at low frequencies no longer propagate along
light cone. If we detect a black hole using these photons, some difficulties
appear because we can not determine the position of event horizon which is
defined by null surface. To overcome these difficulties, the simplest way may
be an effective description by introducing an effective spacetime in which the
photons propagate along the light cone. Then, we find, comparing the results
with those of the original spacetime, that the event horizon and temperature do
not change, but the area of the event horizon and Bekenstein-Hawking entropy
become different. We show that the total entropy for this system, which is
still the same as that of original spacetime, consists of two parts, one is the
Bekenstein-Hawking entropy and the other is the entropy arising from the
coupling of electromagnetic field and Weyl tensor. We also present the
effective descriptions for the Smarr relation and first law of thermodynamics.
