Airy disk

When light passes through a circular aperture, such as a lens or telescope objective, it does not focus to a single point. Instead, diffraction spreads the light into a pattern of concentric rings: a bright central disk surrounded by progressively fainter rings. The central disk is called the Airy disk, named after the astronomer George Biddell Airy, who first described it mathematically in 1835. This pattern is not a flaw but an unavoidable consequence of wave optics, and it determines how closely two objects can be resolved before they blur into one.

The size of an Airy disk depends on the wavelength of light and the diameter of the aperture. The radius of the first dark ring, which defines the edge of the disk, is approximately 1.22 times the wavelength divided by the aperture diameter. In practice, this means that blue light (shorter wavelength) produces a smaller disk than red light, and a larger aperture produces a smaller disk. For a 10 cm telescope observing visible light around 550 nanometers, the Airy disk has an angular width of roughly 1.4 arcseconds. This sets an upper bound on how fine a detail the telescope can distinguish, regardless of magnification or atmospheric conditions.

The Airy disk appears in all imaging systems using circular apertures: telescopes, microscopes, camera lenses, and optical instruments. In microscopy, where resolution is critical, the Airy disk defines the diffraction limit. Two point sources separated by less than the radius of one Airy disk cannot be distinguished as separate objects; they appear merged. This phenomenon is sometimes called the Rayleigh criterion, which states that two objects are just resolved when the center of one Airy disk falls on the first minimum of the other.

Practical implications and measurement

Engineers and scientists must account for the Airy disk when designing optical systems. Attempts to improve resolution by purely optical means have limits; beyond the diffraction limit, tricks like apodization (modifying the aperture shape) can reduce the size of the central disk at the cost of raising the brightness of the rings around it. In astronomy, adaptive optics systems attempt to correct for atmospheric distortion that blurs the Airy disk, effectively sharpening the point spread function. In microscopy, techniques like confocal imaging and two-photon excitation can achieve sub-diffraction resolution, but they work by changing the detection method rather than by eliminating the Airy disk itself.

The Airy disk is easily observed in real life: look at a distant street lamp through a small hole in your hand or through binoculars held at arm's length, and you will see a bright spot surrounded by faint rings. The pattern is stable and repeatable, making it useful for testing optical systems. In laboratory practice, measuring the size of an Airy disk can reveal whether a lens or telescope is performing to specification or whether optical alignment is drifting.