So I understand that similar questions has been asked before. There is a specific aspect I don't understand which I don't think has been explained.
If I shine a light on a surface and then I add a barrier, isn't there still a some photons remaining on the other side of the barrier which will still be travelling toward the same surface. Therefore, wouldn't the shadow moving across the surface be travelling at a slower speed than that of light. I've made a rough illustration of what I mean. (Sorry it's an MSPaint job).