Black hole is a mass concentration which is so strong and so massive that even light cannot escape the gravitational field. So the only thing you can observe is the interaction of the black hole with something around it. There’s gas around the black hole that has not yet fallen in but on its way inward. This material is so hot that it glows, shines and radiates light. This light around the black hole is focused toward the observer. From the observer’s point of view – from our point of view here on earth – it looks like there’s a ring three times larger than the actual black hole – the black hole shadow.