Rayleigh Criterion
The condition for two point sources to be barely resolved by an optical instrument, stating that the central diffraction maximum of one image coincides with the first minimum of the other.
Governing formula
θ_min = 1.22 λ / D
SI unit
rad (radians)
In depth
Sets the diffraction resolution limit for circular apertures of diameter D. Angular resolution improves with larger aperture diameter and shorter wavelength.
Examples in the real world
Calculating the minimum aperture size required for a spy satellite lens to resolve 10 cm features on Earth from orbit.