Rayleigh's criterion

 

According to Rayleigh's criterion, two point source diffraction patterns are just resolved when the center of one pattern coincides with the first dark fringe of the other.

The angular displacement of the two sources q when just resolved is given by....

... where (a) is the diameter of the pupil, telescope objective etc.. Since the angles are small, sin q can be replaced with the angle q in radians.

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