Rayleigh limit

The Rayleigh limit, named after physicist Lord Rayleigh, is a principle in optics that defines the minimum separation at which two adjacent point sources of light can be distinguished by an optical instrument. This limit is due to the diffraction of light, which causes the image of a point source to spread out into a pattern called the Airy disk.12

When two point sources are very close together, their Airy disks overlap. Rayleigh determined that these two sources can be resolved (seen as separate) when the center of one Airy disk falls on the first dark ring of the other. This concept is crucial for understanding the resolving power of optical instruments like telescopes and microscopes, setting a fundamental limit on their ability to distinguish fine details.12