If an aperture is placed between a source of light and a screen, a sharp illuminated region is obtained on the screen. This indicates that light travels in straight line. Newton’s corpuscular theory was able to explain this behaviour. Later on it was proposed that light behaves as a wave. The waves have the property of bending round any comer, as everyone is aware of this property possessed by sound waves. ‘Surely, if light is a wave it must also show this behaviour.
The first indication of bending of light round on edge was observed by Grimaldi. He observed that with a small source of light the shadow of light obstacle was larger than given by geometrical construction it was also observed that shadow was not well defined but consisting of -some coloured fringes near the outer side of the shadow. ·
Thus bending of light round the comer or the departure of light path from ·true rectilinear path was established and phenomenon was called as ‘diffraction. The bright: dark or coloured fringes so obtained are called diffraction pattern. ·
The diffraction phenomenon could not be explained by Newton’s corpuscular theory and Huygen’s wave theory. Next attempt was made by Thomas Young. He considered the diffraction as interference between the direct light and light reflected from the edges of obstacle.
The idea was not accepted because Fresnel showed that the details of the diffraction pattern do not depend ‘upon the material of the obstacle. The correct explanation was provided by Fresnel who considered that diffraction phenomenon is caused by the interference of the innumerable secondary wavelets produced by the unobstructed position of the same wave front.
He was also able to explain the approximate rectilinear propagation of light.
Diffraction phenomenon can be classified into following two classes only on the of positions of source and screen:
(i) Fresnel’s diffraction: In this class either the source or screen or both are at distance from the obstacle and thus distances are important. Here the incident wavefronts are either spherical or cylinderical.
(ii) Frounhofer’s diffraction: In this class both the source and the screen are at distance from the obstacle and thus inclination are important not the distances the wavefront is plane one.