According to Rayleigh’s criterion, two nearby images are said to be resolved if they are separated by at least a certain minimum distance so that the position of diffraction central maximum of the first image coincides with the first diffracted minimum of the second image in any order of diffraction and vice versa.

Rayleigh’s criterion can be explained by considering the diffraction patterns due to two spectral lines of wave lengths *λ*_{1} and *λ*_{2}. Let *A* and *B* represent central maxima of diffraction patterns corresponding to wave lengths and *λ*_{1} and *λ*_{2}. As shown in fig 19(a), the diffraction angle for the central maximum of the image *B* is larger than the diffraction angle for the central minimum of the ...

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