Diffraction is the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. It occurs when waves interact with particles. The diffracting object or aperture effectively becomes a secondary source. In classical physics, the diffraction phenomenon is described by the Huygens–Fresnel principle that treats each point in a propagating wavefront as a collection of individual spherical wavelets. The characteristic bending pattern is most pronounced when a wave from a coherent source, such as a laser, encounters a slit/aperture that is comparable in size to its wavelength.

Diffraction of light occurs when a light wave passes by a corner or through an opening or slit that is physically the approximate size of, or even smaller than that light’s wavelength. A very simple demonstration of diffraction of waves can be conducted by holding your hand in front of a light source and slowly closing two fingers while observing the light transmitted between them. The parallel lines observed are actually diffraction patterns.

Diffraction takes place with sound, electromagnetic radiation such as light, X-rays, and gamma rays, and with very small moving particles such as atoms, neutrons, and electrons, which show wavelike properties. One consequence of diffraction is that sharp shadows are not produced. When a beam of light falls on the edge of an object, it will not continue in a straight line but will be slightly bent by the contact, causing a blur at the edge of the shadow of the object; the amount of bending will be proportional to the wavelength. When a stream of fast particles impinges on the atoms of a crystal, their paths are bent into a regular pattern, which can be recorded by directing the diffracted beam onto a photographic film.