Fraunhoffer Experiment

In optics, Fraunhofer diffraction, or far-field diffraction, is a form of wave diffraction that occurs when field waves are passed through an aperture or slit causing only the size of an observed aperture image to change due to the far-field location of observation and the increasingly planar nature of outgoing diffracted waves passing through the aperture.

It is observed at distances beyond the near-field distance of Fresnel diffraction, which affects both the size and shape of the observed aperture image, and occurs only when the Fresnel number F >> 1, wherein the parallel rays approximation can be applied.

Fraunhofer diffraction utilises the Huygens–Fresnel principle, whereby a wave is split into several outgoing waves when passed through an aperture, slit or hole, and is usually described through the use of observational experiments using lenses to purposefully diffract light. When waves pass through, the wave is split into two diffracted waves travelling at parallel angles to each other along with the continuing incoming wave, and are often used in methods of observation by placing a screen in its path in order to view the image-pattern observed.

When a diffracted wave is observed parallel to the other at an initial near-field distance, Fresnel diffraction is seen to occur due to the distance between the aperture and the observed canvas (s) being more than 1 when calculated with the Fresnel number equation, which can be used to observe the extent of diffraction in the parallel waves through the calculation of the aperture or slit size (a), wavelength and distance from the aperture, (L). When the distance or wavelength is increased, Fraunhofer diffraction occurs due to the waves going towards becoming planar, over the extent of diffracting apertures or objects.