Resolving power of an optical instrument is the ability to separate the images of two objects which are closed together.
E.g. Some binary stars look like a single star when viewed with naked eyes. When the same is viewed through a good telescope, they appear as two distinct stars which are closed together.
Resolving power (RP) of an optical instrument is the reciprocal of minimum separation of two points seen as distinct image seen through the optical instrument.
RP is directly proportional to refractive index of the medium