If an opaque obstacle (or aperture) is placed between a source of light and screen, a sufficiently distinct shadow of opaque (or an illuminated aperture) is obtained on the screen .This shows that the light travels approximately in a straight lines. If, however, the size of obstacle (or aperture) is small (comparable with the wavelength of light), there is a departure from straight line propagation and the light bends round the corners of the obstacles(or aperture) and enters the geometrical shadow. The bending of light at sharp corners/edges is called diffraction. As a result of diffraction, the edges of the shadow (or illuminated region) are not sharp , but the intensity is distributed in a certain way depending upon the nature of obstacle (or aperture).

Let us first explain, how the light bends around a sharp corner. According to Huygen's principle, when a wave propagates, each point on its wave front serves as the source of spherical secondary wavelets having same frequency as that of original wave [Fig. 1a]. The resultant at any point afterward is the envelope of these secondary wavelets. However, this picture does not explain the diffraction of light through small apertures.

If we assume that, as shown in fig.1b, each unobstructed point of a wavefront, at a given instant, serves as a source of spherical secondary wavefront, the amplitude of the optical field at any point beyond is the superposition of all these wavefronts. The maximum path difference between these secondary wave fronts at any point P is equal to AB . (Path difference will be equal to AB if point P merges with either point A or point B) .