Term Diffraction

Introduction
The term diffraction in the case of waves refers to their bending round the obstacles. When the obstacle is large compared to the wavelength no wave bends around the edges of the obstacle. When the size of the obstacle is small compared to the wavelength of the light waves bend round the edges of the obstacle. When the size of the obstacle is very very small the waves bend round it so that we find no practical effect on the wave. The diffraction phenomena is more predominant when the size of the obstacle is small and is comparable with the wavelength of the incident light.
                                  
 One of the examples of diffraction phenomena is that when a beam of light passes from a narrow slit it spreads out to certain extent in the geometrical shadow.
                                               
In the above figure an obstacle AB with a straight edge is place in the path of a light wave spreading from a narrow slit illuminated by a monochromatic light source. The straight edge A is parallel to the slit S. The geometric shadow of edge A on the screen C is not sharp. A small portion of the light bends around the edge A into the geometrical shadow below the point C. Intensity gradually decreases as we enter into the shadow below C. As we go above C, the intensity alternately increases and decreases; several bright and dark bands parallel to the edge are observed. These bands are called diffraction pattern. The width of these bands goes on decreasing as we go upwards and uniform illumination is observed farther away from C

Term Diffraction : Pattern

                        
    A very small circular disk of diameter AB obstructs the path of (rays) waves emerging from a point source S. The diffraction pattern is observed on the screen SC. If the light propagates in straight line there would be a shadow of diameter CD on the screen. If the distance between the disk AB and the screen CD is great enough, we find diffraction pattern consisting of alternating dark and bright rings with a bright circular spot at the centre at 'O'.

Term : Diffraction

The diffraction phenomena is observed when the condition l `~~` b² / 4`lambda`  is satisfied, where l is the distance between the object and the screen, b size of th object an d`lambda`  is the wavelength of light obstructed by the object. There is a little difference between the formation of interference and diffraction patterns, though superposition of waves is involved in both the cases. Interference is the result of superposition of light waves emitted by two or more number of separate coherent sources, where as diffraction is due to superposition of light wavelets originating from every point of a wavefront which act as infinitely small coherent sources. Diffraction effects are observed only when a portion of the wave front is obstructed by the obstacle.