Gravitational lensing is a phenomenon in which light from a distant source is bent or distorted by the gravitational field of a massive object, such as a galaxy or a black hole. While gravitational lensing can alter the path of light, it does not involve the concept of a refractive index or the slowing down of light in the same way as materials like water or glass.
In the case of materials with refractive indices, such as water or glass, the speed of light is indeed reduced as it passes through the medium due to interactions between the photons and the atoms or molecules of the material. This reduction in speed is described by the refractive index of the material.
In gravitational lensing, the bending of light is a consequence of the curvature of spacetime caused by the massive object. This curvature is described by Einstein's theory of general relativity. However, the speed of light itself is not altered by gravitational lensing. Light still travels at the speed of light in a vacuum (approximately 299,792,458 meters per second) as it passes through gravitational fields.
Therefore, while gravitational lensing can change the path and direction of light, it does not involve the slowing down of light or the concept of a refractive index as seen in transparent materials.