CBSE Class 12-science Answered

Diffraction can be explained by Huygens -  Fresnel principle . According to this principle each point on a wavefront , which is unobstructed , acts as a source of secondary waves . The secondary sources are mutually coherent and the waves emitted by them interfere . It should be remembered that the diffraction effects are observed only when a portion of the wavefront is cut off by some obstacle .

The amplitude t a point  P due to the secondary wave depends on the primary wavefront and the line joining the wavefront and the point P . 

S is the primary wavefront . dS is an elemental area which acts as a secondary source , n is the normal to the primary wavefront at dS , r is the distance between the secondary source and the point P . is obliquity , is the angle between the normal n and r as shown in figure .

The effect of the whole wavefront of area S at point P is given by the sum of oscillations due to individual elemental areas dS to the wavefront is divided .

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