Q.What is Rayleigh's criterion?

Diffraction limits the sharpness of any image formed by a finite-sized aperture: even a perfect lens or mirror images a single point source not as a point but as a small central bright disc (an Airy disc) surrounded by faint concentric rings, because the circular aperture itself diffracts light, with the central disc's half-angular spread given by $a\theta=1.22\lambda$ (i.e. $\theta=1.22\lambda/a$, the 1.22 factor arising from the specific mathematics of a circular, rather than rectangular, aperture). When two point objects lie close together, their two Airy-disc images can overlap and blur into one; Rayleigh's criterion defines the two objects as 'just resolved' when the central maximum of one image's diffraction pattern falls exactly on the first minimum of the other's, corresponding to a minimum resolvable angular separation of $\theta=1.22\lambda/a$ (radians). Resolution is the smal …