You need to read up on the subject. The photon has a dual nature. It is both a wave and a particle, but depending on how it is measured it can appear as one or the other. If you find that one of the two is behaving as a particle then, you will find that the other is also.
How are you violating Einstein's postuate? The speed of light in a vacuum is c, but the velocity is slower through matter, like the glass or plastic of a fiber optic cable.