Rayleigh's Criterian:

According to Rayleigh's Criterian the two illuminated objects lying close to each other are said to be resolved, if the central maximum of diffraction pattern of one falls on the first secondary minima of the other.

Resolving power of grating

The resolving power of a grating is defined as the smallest wave length difference dλ for which the spectral lines can be first resolved at the wavelength λ. it is mathematically given as:

R.P = λ/dλ