Chapter32 Relativity Suppose an event is described by two observers, one in frame s(地面) and the other in frame s'(火 4), S' is moving with constant velocity v relative to s when a pulse of light leaves a light source in frame sSuppose an event is described by two observers, one in frame S（地面） and the other in frame S’ （火 车），S’ is moving with constant velocity v relative to S. when a pulse of light leaves a light source in frame S’: