Black holes are objects with such strong gravity that not even light can escape. This is because the escape velocity of a black hole is greater than the speed of light. The escape velocity is the speed at which an object needs to travel in order to escape the gravitational pull of a body. The speed of light is the fastest speed that anything can travel in the universe.

We know that light can't escape a black hole because of the way that black holes interact with light. When light passes near a black hole, it is bent and distorted by the black hole's gravity. This is called gravitational lensing. If light could escape a black hole, we would be able to see the light that is bent and distorted by the black hole's gravity. However, we don't see this light. This is because the light is too redshifted by the black hole's gravity. Redshift is the phenomenon where light is stretched out in wavelength as it travels through a strong gravitational field.

The amount of redshift that light experiences is directly proportional to the strength of the gravitational field. In the case of a black hole, the gravitational field is so strong that the light is redshifted to the point where it is no longer visible. This is why we can't see light that is emitted from inside the event horizon of a black hole.

The event horizon is the boundary around a black hole where the escape velocity is equal to the speed of light. Once something passes the event horizon, it is no longer possible for it to escape the black hole's gravity. This means that anything that falls into a black hole, including light, is trapped forever.

The fact that light can't escape a black hole is one of the most amazing and mysterious things about these objects. It is a reminder of the power of gravity and the strange and wonderful things that can happen in the universe.