Lord Rayleigh, a 19th century physicist, showed that a telescope optic would be sensibly indistinguishable from a theoretical perfect optic if the light (strictly, the wavefront) deviated from the ideal condition by no more than one quarter of its wavelength.

The rule developed by lord Rayleigh that if the difference between the longest and the shortest paths leading to a selected focus is less than or equal to one-quarter of the wavelength, then the imaging quality is nearly indistinguishable from perfect.

An arbitrary (but useful) criterion for resolution. For a microscope, this is that the resolution is equal to 1.22· NA, where λ is the wavelength of the light and NA is the numerical aperture of an objective.