Diffraction

The spreading out of electromagnetic radiation as it leaves an aperture. The angle of spread, which cannot be eliminated by focusing, is proportional to the ratio of the wavelength of radiation to the diameter of the aperture.

The deviation of an electromagnetic wavefront from the path predicted by geometric optics when the wavefront interacts with, i.e., is restricted by, a physical object such as an opening (aperture) or an edge. Note: Diffraction is usually most noticeable for openings of the order of a wavelength. However, diffraction may still be important for apertures many orders of magnitude larger than the wavelength.