Fresnel and Fraunhofer diffraction
Advanced
Near-field and far-field diffraction regimes.
Fraunhofer (far-field) diffraction has parallel waves and simple patterns; Fresnel (near-field) has curved wavefronts. The Fresnel number a²/λL tells which applies.
Single slit with a screen far away is Fraunhofer.
Poisson spot is a Fresnel effect.
In more detail
Fresnel diffraction occurs when the source or screen is near the aperture, producing complex patterns. Fraunhofer diffraction occurs at large distances, and it has simpler, well-defined patterns.
Real-world uses
- Fraunhofer patterns are used in spectrometers and gratings.
- Fresnel zone plates focus light and X-rays.
- Engineers designing sensors use the Fresnel and Fraunhofer regimes to predict diffraction patterns at different distances.
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Frequently asked questions
How can I tell which applies?
If the distance is large compared with the aperture size squared over the wavelength, Fraunhofer applies.
Are Fraunhofer patterns easier to calculate?
Yes, they are Fourier transforms of the aperture.
What is the difference between the two?
Fraunhofer diffraction applies in the far field, while Fresnel diffraction applies closer to the obstacle.
Related topics in Optics
Coherence and lasers · Holography · Fermat's principle · Fibre optics numerical aperture · Colour and the eye · Additive and subtractive colour mixing