Resolving power is defined as the inverse of the distance or angular separation between two objects which can be just resolved when viewed through the optical instrument.
For microscopes, the resolving power is the inverse of the distance between two objects that can be just resolved. This is given by the famous Abbe’s criterion given by Ernst Abbe in 1873 as
△ d = λ/2 n sin θ
Resolving power = 1/△ d = 2n sin θ/λ
Where n is the refractive index of the medium separating object and aperture.
(a) The resolving power increases when the wavelength of the incident light decreases.
(b) The resolving power decreased when the aperture of the objective lens is decreased.