Classes Of Diffraction
classes of Diffraction: The difftraction phenomenon are conveniently divided into two general classes:
-
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) . - 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.
Diffraction by a narrow single slit
Consider a single slit Fig. 2a(a rectangular aperture, whose length is larger compared to its breadth, and breadth is quite narrow, comparable to the wavelength of light 
0.1 mm for visible light) placed in front of a monochromatic light source as shown in figure 2(b).
When a plane wave front is incident on the slit, each point on the wave front acts as the source of spherical secondary wave fronts. Lets divide the wave front incident on the slit in a large number of elements, each of width ds, (infinitesimaly small).
The part of each secondary wave, originating from these small elements and travelling normal to the plane of slit will be focussed at P0 while those travelling at an angle 
will reach P (Fig.2b). Considering first the wavelet emitted by the element ds situated at the center of slit (let us call it origin). Its amplitude will be directly proportional to length ds and inversely proportional to the distance x (it will produce spherical wave front). At P, it will produce an infinitesimal amplitude (electric field) which is expressed as
where 'a' is the amplitude of incident wave, 
and 
are its frequency and wave vector respectively.