If there is no mass for the light, it should penetrate. But light is not penetrating through solid medium. If the speed of light is constant there is some work should be done to maintain the speed. Einstein said that the light is curved because of Sun's gravity, to bend that also work has to be done. Maybe the black hole is completely dark solid matter that's why the light was unable to penetrate?
This question is in my mind for years please explain
One can start from photons which are elementary particles and have zero rest mass.
It can be shown that light is a superposition of photons . The double slit experiment one photon at a time demonstrates this, as the accumulation of photons shows the interference pattern expected from light at that frequency.
Two photons, if not completely colinear, will have an invariant mass, given by the length of their added four vector. A real example is the pi0 which is a hadron that decays into two photons. Its mass is 135 MeV and the four vector of the ensemble of two photons will have the same mass due to energy and momentum conservation in decays.
Thus light, which is an ensemble of photons may have an invariant mass if the photons have a small angular dispersion around the classical ray direction. One has to define what one means when talking of "light".
