Optics 101

Fresnel vs Fraunhoffer Diffraction

Aug 1 2014

Diffraction is the slight bending of light as it travels around the edges of an object/slit or after passing through an opening. The amount light is bent by depends on the exact wavelength of the light and the size of opening.

Diffraction can be usually be characterised by one of two types:

Fresnel diffraction
Fraunhoffer diffraction

Fresnel Diffraction

Fresnel diffraction, or near-field diffraction, occurs when a wave passes via a small hole and diffracts, creating a diffraction pattern. The size of the pattern depends on the distance between a projection and an aperture.

This type of diffraction occurs when the distance of propagation of diffracted waves is short, resulting in a Fresnel number greater than 1. Diffracted waves become planar when the distance of propagation is increased.

Fraunhofer Diffraction

Fraunhofer diffraction, or far-field diffraction, is a type of wave diffraction which occurs when field waves pass via a slit or an aperture, thereby changing the size of an observed aperture image with respect to the increasingly planar nature of diffracted waves and far-field location of observation.

Comparison of Fresnel and Fraunhoffer Diffraction

Characteristics	Fresnel Diffraction	Fraunhofer Diffraction
Surface of calculation	Fresnel diffraction occurs on spherical surfaces.	Fraunhofer diffraction patterns on flat surfaces.
Diffraction pattern	Shape and intensity of diffraction pattern change as the waves propagate downstream of the scattering source.	Shape and intensity remain constant.
Movement of diffraction pattern	Move along the corresponding shift in the object.	Remains in a fixed position.
Observation distance	Screen placed at finite distance from an object.	Infinite observation distance.
Wave fronts	Cylindrical wave fronts.	Planar wave fronts.

References

Differences between Fraunhofer Diffraction vs Fresnel Diffraction? – Function Space
Diffraction And Its Types – Padante.com

Comments

David Spector says:

July 30, 2021 at 8:37 AM

Discussions of Fresnel vs Fraunhofer diffraction typically ignore the basic double-slit quantum mechanics experiment. I assume that this experiment is far-field, and therefore that the pattern obtained when both slits are open is the Fraunhofer diffraction pattern, which is simpler and slower changing with changes in experimental geometry.

Also ignored are the geometric modifications that must be made to printed circuit masks as the wavelength of the illumination approaches zero. In this case, an inverted diffraction pattern must be used. That is, the pattern must yield straight edges with little or no diffraction when illuminated. I'm not sure how this is analyzed, or how it relates to the two kinds of diffraction patterns.

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