Perfect converging lens. Object is imaged by refraction to a real image distant from the focal point in Image Space.

Resolution: Minimum distance between two points where they can be distinguished as separate. This is the Raleigh Criterion, and is defined as the minimum distance separating two adjacent Airy discs.

Rayleigh, therefore based his definition of resolution on the diffraction of light, wavelength, and NA (which was defined later of course) by the imaging lens system.

Abbe, on the other hand, based his definition of resolution only on the objective NA and wavelength.

A point object will be imaged as a series of concentric rings due to wave interaction (constructive and destructive interference). The central maximum (diameter = d) is termed the Airy disc.

Two closely spaced objects may be resolved only if their Airy discs are sufficiently far apart. As the objects are placed closer together (A–C), the resulting diffraction patterns (Airy discs) merge into one, non-resolvable, central pattern (D).

**The Rayleigh criterion, defined as the minimum resolvable detail, when the first diffraction minimum of the image of one source point coincides with the maximum of another (C). **

d=min distance; l = wavelength of light; *n*=the refractive index of the medium between the sample and lens; a or alpha=the Angular Aperture (of the microscope objective). Recall that NA=*n*sin(Alpha/2)

d=min distance; f=focal length of the lens; l = wavelength of light, D=diameter of the (telescope) lens.