The measured value of the speed of light is about 300,000 km/s. suppose a futuristic space train is traveling at 200,000 km/s with its headlights on. if you could measure the speed of the light from the headlights, you would find it to be
the speed of light generally would be 300000km/s but since the train is moving in the same direction as the light it would apparently appear to be 100000km/s
