Fresnel diffraction

Fresnel diffraction is a near-field diffraction regime in which finite propagation distance makes wavefront curvature significant. Its patterns can change with distance from the diffracting object.

Huygens–Fresnel principle: A wave-propagation principle that treats each point on a wavefront as a source of secondary wavelets. It provides the wavelet-based construction underlying Fresnel diffraction.
Wavefront: A surface of constant phase in a propagating wave. Fresnel diffraction depends on how the wavefront's curvature evolves after an aperture.
Fresnel zone plate: A lens-like optical element made of concentric rings that alternately transmit or block light. Its ring radii are chosen to focus light through Fresnel-zone interference.
Fraunhofer diffraction: The far-field diffraction regime in which wavefront curvature across the observation region can be neglected. It is the limiting regime where Fresnel curvature effects become negligible.
Fresnel integral: An integral that calculates a propagated optical field using a quadratic approximation to its phase. It predicts the field when curved wavefronts must be retained.
Optical path length: The distance light would travel in vacuum during the time taken to traverse a medium. Path-length differences determine phase differences among contributions to the pattern.
Fresnel diffraction in imaging: The use of finite-distance diffraction models to predict image formation and propagation effects. It describes edge and aperture effects when imaging distances are not in the far field.
Fraunhofer approximation: A diffraction approximation that replaces propagation phase by a linear dependence on aperture position. Unlike Fresnel propagation, it omits the quadratic phase responsible for near-field curvature effects.
Fresnel number: A dimensionless ratio comparing aperture size squared with wavelength times propagation distance. Its value indicates whether finite-distance curvature significantly shapes the pattern.
Wavelength: The spatial period of a wave, measured between successive points of equal phase. It sets the scale of phase variation and enters the Fresnel number.

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