The Rayleigh criterion is a principle in optics that defines the minimum angular separation at which two point sources of light can be distinguished as separate entities. According to this criterion, two sources are resolvable if the central maximum of one source's diffraction pattern coincides with the first minimum of the other. This concept is crucial in fields like astronomy and microscopy, where resolving power is essential for observing fine details. The criterion is named after the British scientist Lord Rayleigh, who formulated it in the 19th century. It highlights the limitations imposed by diffraction, which occurs when light waves encounter obstacles or apertures. Understanding the Rayleigh criterion helps in designing optical instruments, such as telescopes and cameras, to improve their ability to resolve closely spaced objects.