The difftraction phenomenon are conveniently divided into two general classes:

1. Fraunhoffer Diffraction : When both the source of light and the screen where diffraction pattern is observed are at very large distances (distances >> ) from the obstacle (aperture) .
2. Freshnel Diffraction : When, either or both, source of light and screen on which the diffraction pattern is observed are at finite distance from the obstacle (aperture).

In the following lectures we shall be studying the Fraunhoffer diffraction phenomenon occuring due to narrow slit, double slit and grating (composed on large number of narrow slits). We shall also be discussing the resolving power of a grating and other image forming systems.

Fraunhoffer Diffraction : In the case of Fraunhoffer diffraction, both the source and slit are very far from the diffracting aperture. In other words, this is equivalent to saying that the light originating from the source (where it has a spherical wavefront) when reaches at the aperture, the wavefront is nearly plane. The intensity of light at each point of aperture is thus nearly same. (If the distance between source and slit is not large, the wavefront at the aperture will be spherical and intensity of light at each point of aperture will be diffrent ).

After diffraction, each point of wavefront at aperture, will act as a new source of light and will spread as spherical wavefront .When the screen is very far from the aperture, the wavefront reaching the screen wiil again be planar and the wavefronts converging on any point on the screen will have same amplitude, and thus we need not worry about difference in field strength from different wavefronts.