The maximum resolution of an optical system at a given wavelength, which is set by the wave nature of light (see diffraction). It is usually expressed in seconds of arc. An optical system is said to be diffraction-limited when it meets the Rayleigh criterion. This occurs when there is no more than a quarter of a wavelength of light between the wavefronts reaching the focal point from all parts of the aperture. However, in practice the resolving power of a telescope is usually better than this theoretical limit, so optics that meet the Rayleigh criterion are not necessarily perfect. Large modern telescopes have diffraction limits of better than 0.1 arcsecond, but because of the effects of atmospheric seeing such resolutions can be achieved only by the use of adaptive optics. See also resolving power.