““In order to show that not even light can escape from the gravitational pull of a black hole, we have to hone the argument further. Light, as we know, always moves at the same velocity. When it rises against gravity, light, will therefore not slow down. Unlike a bullet, it cannot come to rest and turn around. But by rising it will lose energy and thereby its wavelength will increase. Thus the light becomes dimmer and dimmer-with the result that outside the sphere at which bullets turn around the light cannot be seen anymore. We may therefore conclude that light, just as bullets, cannot leave the gravitational field of a black hole.””
