Rayleigh's Criterion

Rayleigh’s Criterion defines the minimum separation at which two light sources can be distinguished as separate entities by an optical system like a telescope or microscope. When two sources are closer than this limit, their light patterns overlap so much that they appear as a single source. The criterion considers the diffraction of light—its natural spreading—creating a boundary for resolving detail. Essentially, it provides a standard for the finest detail an instrument can distinguish, balancing the system’s optics and the wavelength of light used. This helps scientists understand and improve the resolving power of optical devices.