Diffraction  of light The bending of light around an obstacle. Consider a narrow slit, illuminated by a light beam. On a screen kept  in  the  front  of the slit, light spreads in the region of shadow,  forming  bands of maxima and minima. This occurs due to interference  of  the  secondary  wavelets  originating  from the wave front at the slit. (see Huygens theory).

Diffraction phenomena are studied under two classes.

1. Fraunhofer class - in which the source and the screen are both effectively at infinity from the slit causing the diffraction. Such distances are realized in practice using lenses.

2. Fresnel  class - in which either the source or the screen or both are at finite distance from the slit.

Fraunhofer  class is a special case of Fresnel class because in  the  former  the wave front is plane, while for the latter the wavefront  may  have any shape.