Ans.
In optics, Fraunhofer diffraction (named after Joseph von Fraunhofer),
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 \ll 1, wherein the
parallel rays approximation can be applied.
On the other hand, Fresnel diffraction or near-field
diffraction is a process of diffraction that occurs when a wave passes through
an aperture and diffracts in the near field, causing any diffraction pattern
observed to differ in size and shape, depending on the distance between the
aperture and the projection. It occurs due to the short distance in which the
diffracted waves propagate, which results in a Fresnel number greater than 1 (F
> 1). When the distance is increased, outgoing diffracted waves become
planar and Fraunhofer diffraction occurs.