Fig. 28. a. Intensity distributions for two unresolvable points separated by an angle less than 1.22λ/d; that is, less than the radius of the disc. The dashed line represents the combined intensity of the two patterns. b. Intensity distributions for two just resolvable points separated by an angle of 1.22λ/d (Rayleigh criteria). Separation equals one disc radius; the peak of one pattern falls on the first minimum of the other pattern. The dashed line shows a dip in the combined intensities that is sufficient for resolving the two sources.