undiffracted

Undiffracted is an adjective used to describe a wave—such as light, sound, or matter waves—that travels without undergoing diffraction, or with negligible diffraction, as it passes through an aperture or past an obstacle. Diffraction is the bending and spreading of waves when they encounter boundaries or openings; an undiffracted wave would continue primarily in its original direction with little angular spreading.

In practical terms, most waves experience some diffraction when they interact with finite-sized features. The concept of undiffracted light often appears in the context of approximations or specific components of a wave field. For example, in diffraction analysis the zero-order or undiffracted beam is the portion that travels without angular deflection, while higher orders are diffracted to various angles. The undiffracted component is more readily treated in the limit where the wavelength is small compared with the features involved, a regime described by geometric optics.

The term is used as a descriptive aide rather than a distinct physical phenomenon. It helps distinguish a primary, forward-propagating component from diffracted portions in experiments and engineering applications, such as imaging systems, diffraction gratings, and wave-propagation simulations. In some fields, “unscattered” or “undiffracted” may be used to refer to the direct beam that remains essentially unchanged, with diffracted waves representing the redistributed or redirected portions.

See also: diffraction, undiffracted vs diffracted components, zero-order beam, Fraunhofer and Fresnel diffraction.