I never understand why the speed of light is the same for all observers irrespective of their motion relative to the source of light. Now suppose I am sitting at the back of a vehicle which is travelling at the speed of 0.999999999999c and light approaches me from behind the vehicle. i.e. I am going away from the source of light while I can still see the light. Now I attach an instrument to my car for measuring the speed of light. Won't it measure 0.000000000001c. Please help me understand why will the instrument still record 0.999999999999c and not 0.000000000001c according to the theory of relativity.