Electromagnetic waves travel mostly in vacuum medium, in outer space, but sometimes in gaseous media, such as in gaseous atmospheres involved in nebulae. If electromagnetic-waves come anywhere in the vicinity of a black-hole, the whole of it enters the black-hole. Can we suppose, that the event-horizon of the black-hole acts in a way like an interface, separating two optical media with different refractive indices : vacuum (the medium from which EM waves are "incident"), and the unknown medium beyond. If, this supposition holds true, then, we would know one thing at least regarding this unknown medium, that, EM waves travel in this medium with a velocity > c, as this medium is rarer, than vacuum itself.
(Question of an amateur)