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For a model of the electromagnetic field coupled to Weyl tensor, Maxwellequations are modified and photons at low frequencies no longer propagate alonglight cone. If we detect a black hole using these photons, some difficultiesappear because we can not determine the position of event horizon which isdefined by null surface. To overcome these difficulties, the simplest way maybe an effective description by introducing an effective spacetime in which thephotons propagate along the light cone. Then, we find, comparing the resultswith those of the original spacetime, that the event horizon and temperature donot change, but the area of the event horizon and Bekenstein-Hawking entropybecome different. We show that the total entropy for this system, which isstill the same as that of original spacetime, consists of two parts, one is theBekenstein-Hawking entropy and the other is the entropy arising from thecoupling of electromagnetic field and Weyl tensor. We also present theeffective descriptions for the Smarr relation and first law of thermodynamics.

Detect black holes using photons for coupling model of electromagnetic and gravitational fields