why back holes are invisible
Answers
Black Holes are invisible because they are so dense. Their extreme density means that their gravity is so strong that not even light can escape.
Although we cannot see black holes, we can detect or guess the presence of one by measuring its effects on objects around it.
Many black holes have objects around them, and by looking at the behavior of the objects you can detect the presence of a black hole. You then use measurements of the movement of objects around a suspected black hole to calculate the black hole's mass. What you look for is a star or a disk of gas that is behaving as though there were a large mass nearby. For example, if a visible star or disk of gas has a "wobbling" motion or spinning AND there is not a visible reason for this motion AND the invisible reason has an effect that appears to be caused by an object with a mass greater than three solar masses (too big to be a neutron star), then it is possible that a black hole is causing the motion. You then estimate the mass of the black hole by looking at the effect it has on the visible object.
When material falls into a black hole from a companion star, it gets heated to millions of degrees Kelvin and accelerated. The superheated materials emit X-rays, which can be detected by X-ray telescopes.
The surface gravity of black hole is so high that object with the highest speed t can not escape from its surface.That means light with 300,000 KM/second can not escape from the black hole..So we can not see it we see objects with its own light or reflected light