Airy pattern
The Airy disk together with the series of concentric rings that surround it.
Airy pattern: the diffraction limit made visible
An Airy pattern is the image formed when light passes through a circular aperture, such as a lens or telescope objective. It consists of a bright central disk surrounded by progressively fainter concentric rings, all caused by diffraction at the aperture edge. The pattern appears whenever you image a point source of light, and its size directly determines the finest detail your optical system can resolve.
The central bright region, called the Airy disk, contains roughly 84 percent of the light energy. Its angular diameter is given by 1.22 times the wavelength divided by the aperture diameter. For a visible-light telescope with a 100 mm objective observing green light at 550 nanometers, the disk spans about 1.3 arcseconds. The first dark ring appears at 2.44 times this fundamental measure. The surrounding rings drop in intensity very quickly, with the first bright ring receiving only about 1.7 percent of the peak intensity.
Recognition and practical limits
When two point sources lie closer than the Airy disk diameter, their patterns overlap and you cannot distinguish them as separate objects. This is the Rayleigh criterion, the standard definition of optical resolution. A larger aperture produces a smaller Airy pattern and therefore better resolving power. Longer wavelengths produce larger patterns, which is why infrared instruments need bigger optics than visible-light ones to match the same resolution.
The pattern's intensity distribution follows a squared Bessel function of the first kind. In practice, you observe Airy patterns when viewing stars through telescopes, when imaging small defects under microscopes, or in photographic enlargements of sharp edges. In optical systems with aberrations, the rings become asymmetric or irregular; perfect circular rings indicate good optical quality and alignment.
The pattern is named after George Biddell Airy, who calculated its mathematical form in 1835. It appears in every optical instrument that uses a circular aperture, from a simple magnifying glass to the largest astronomical observatories. Understanding the Airy pattern's size is essential when specifying optical components or predicting what an imaging system can actually see.