11ac1064_ions___Astronomy___Black_Holes__Question
[Date Answered] September 16, 2008 [Questioner (age, location)] priyanka (india) [Answer] This limit is known as event horizon. This is the boundary within which the black hole's escape velocity (minimum velocity to escape a gravitational field) is greater than the speed of light. In other words, within this radius a radiation will have to moves faster than light to escape, but because the speed of light in a vacuum is the speed limit, nothing can escape from inside the event horizon of a black hole!. [Question] 
why can't light escape blackhole only within a particular radius?
Quickie Questions - Astronomy - Black Holes