What is Rayleigh’s criterion of resolution? Define resolving power of grating?

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According to the Rayleigh’s criterion, two closely spaced point sources of light are said to be just resolved by an optical instrument only if the central maximum in the diffraction pattern of one Falls over the first minimum in the diffraction pattern of the other and vice versa.

The resolving power of a grating is defined as the ratio of the wavelength of any spectral line to the difference in the wavelength between this line and a neighbouring lines such that two lines appear to be first resolved.

`∴R.P=λ / λ+dλ`

Where,

λ = wavelength of a line

λ+dλ = wavelength of the next line that can just be seen s separate.