If we think of it in terms of classical electromagnetism, a ray of light moving away from a black hole has to climb out of the intense gravitational field and be proportionately red shifted as it loses energy to the gravitational field. Light cannot escape from the event horizonn of a black hole because it will be infinitely red shifted I.e. it's wavelength becomes infinite and conversly its frequency of its EM field displacement is zero I.e there is no wave motion. To put in simpler terms a wave with a freuency of zero is not a wave and so it cannot propagate as per classical EM and so light from the event horizon (or below it) cannot escape from that point.
If I remember correctly I have written about this before on this board but in a bit more detail (i think) if any one is inclined to search for that post.
Edit: here is the post I refer to <[grahamhancock.com];
Edited 1 time(s). Last edit at 11-Apr-19 19:37 by JonnyMcA.