Movie 4 (:09)

Script : A flash does not instantly illuminate the interior, but slowly travels through the tram. As the light must travel out and back, the illuminated region moves outward at half the speed of light.

 

Comments : In this scene, imagine you're holding a camera which sets off a flash inside the tram. The interior NOT lit all at once.

The flash appears from your point of view to move outward at half the speed of light, because the light is travelling out, reflecting off an object, then travelling back to you : twice the distance to the object you're actually seeing at any particular moment.