![Invariance of the speed of light[Credits : Encyclopædia Britannica, Inc.]](http://media-2.web.britannica.com/eb-media/43/92943-004-EC89C42D.gif)
Arrows shot from a moving train (A) and from a stationary location (B) will arrive at a target at different velocities—in this case, 300 and 200 km/hr, respectively, because of the motion of the train. However, such commonsense addition of velocities does not apply to light. Even for a train traveling at the speed of light, both laser beams, A and B, have the same velocity: c.
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