The fundamental physical limit to the resolution of an optical system due to the wave nature of light.The diffraction limit is the point beyond which an ordinary optical system cannot distinguish two nearby objects, no matter how perfect its lenses are. It arises because light behaves as a wave: when light passes through a circular aperture such as a lens, it spreads out slightly, and a point of light is imaged not as a point but as a small blurred disc surrounded by faint rings. If two such discs overlap too much, they merge into one and the eye or camera cannot tell them apart. The practical consequence is that a conventional microscope cannot resolve details much smaller than roughly half the wavelength of the light used — about 200 nanometres for visible light. This limit is not a matter of engineering quality; it is built into the wave nature of light itself. It matters because it long defined the boundary between what microscopists could see and the molecular-scale world of cells, and understanding it was the prerequisite for the super-resolution techniques that eventually broke through it.
The criterion for resolving two images was formulated by Lord Rayleigh in the late nineteenth century, building on earlier work by Ernst Abbe, who in 1873 derived the resolution limit for microscopes in terms of wavelength and numerical aperture.
The blur comes from diffraction at the aperture of the objective lens. Light from a single point source is spread into a characteristic pattern called the point spread function — a bright central spot with concentric rings. Two point sources are considered just resolvable when their central spots are separated by roughly the radius of that first dark ring, a criterion formalised in the Rayleigh criterion. Because the size of the pattern scales with wavelength and inversely with the aperture's diameter, shorter wavelengths and larger apertures give finer resolution, but the pattern never shrinks to a true point.
It sets the design target for every conventional microscope objective and is the benchmark that super-resolution fluorescence microscopy methods such as STED and single-molecule localisation are measured against.
Because the limit scales with wavelength, electron microscopes achieve atomic resolution partly by using electrons with wavelengths thousands of times shorter than visible light.