Abbe's law of limiting resolution

Abbe’s law of limiting resolution

A law for a periodic structure of units separated by distance d and obliquely illuminated by the unrefracted ray and one of the two diffracted rays (extremely oblique illumination). Abbe applied the law of diffraction:
d = 0.5 λ/NA, where λ = wavelength of the monochromic light or shortest of mixed wavelengths, and NA = the limiting numerical aperture of the objective or condenser.
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