From the TED-Ed YouTube channel:
Light always travels at a speed of 299,792,458 meters per second. But if you’re in motion too, you’re going to perceive it as traveling even faster — which isn’t possible! In this second installment of a three-part series on space-time, CERN scientists Andrew Pontzen and Tom Whyntie use a space-time diagram to analyze the sometimes confounding motion of light.
Lesson by Andrew Pontzen and Tom Whyntie, animation by Giant Animation.