Hello. I have little knowledge on relativity but I have read the fundamental concepts. What has been confusing me was the following problem: If a photon A is traveling towards another photon B which is traveling directly into photon A (a head on collision). What will be the speed of photon B relative to photon A? Since the relative speed cannot be two times the speed of light as nothing can surpass the speed of light, could someone please explain this to me?