Diffraction of electromagnetic radiation refers to various phenomena that occur when a light wave encounters an obstacle or opening.

- Diffraction and interference are phenomena associated with all kinds of waves. Electromagnetic waves are a special case however because of their unique behaviour.
- Diffraction deals with the way light bends around the edges of obstacles into regions that would otherwise be in shadow.
- Interference deals with the way that light behaves during the diffraction process.
- Diffraction can be produced by the edges or by a hole (aperture) in any opaque surface or object.
- Diffraction causes a propagating wave to produce a distinctive pattern as light waves interfere with one another. The resulting pattern becomes visible if diffracted light subsequently strikes a surface.
- Diffraction produces a circular pattern of concentric bands when a narrow beam of light passes through a small circular aperture and then strikes a flat surface.

- In classical physics, an explanation of the diffraction of electromagnetic waves treats each point at which a propagating wavefront encounters the edge of an obstacle as a site at which a new spherical wavelet supersedes the original waveform.
- Separate spherical wavelets bend independently of one other beyond the site at which an obstacle is encountered. However, interference between them alters the way they bend and the distance they must travel before striking a surface.
- Explanations that describe the process of diffraction and interference patterns belong to Wave Theory and are the result of more than two centuries of study in the field of optics.

- In modern quantum mechanics, diffusion is explained by referring to the
*wave function*and*probability distribution*of each photon of light as it encounters the corner of an obstacle or the edge of an aperture. - Wave functions and probability distributions are part of mathematical formulations of the outcomes of all possible measurements of a photon’s behaviour in the course of diffraction.