TABLE OF CONTENTS The formation of interference fringes on the screen can be explained on the basis of wave theory. When light passes through S, spherical waves spread out from S. According to Huygens' principle, each point of a wavefront becomes the source of a secondary disturbance and the secondary wavelets emanating from these points spread out in all directions. Hence spherical waves also spread out from the closely spaced holes S 1 and S 2. The radii of these secondary wavefronts increase as they move away from S 1 and S 2 and hence they superimpose more and more on each other. At points where a crest falls over another crest or over a trough, the vibrations are large and the resultant amplitude is greatest; hence the resultant intensity at these points is increased. These points are shown by solid dots. This is the case of constructive interference. At points where a crest of one falls over the trough of the other, the intensity is minimum. This is the case of destructive interference. The points, where the intensity is minimal, are dark and represented by hollow dots in Figure 2. Thus we get alternate bright and dark lines at the intersection of the screen with antinodal and nodal planes. These dark and bright regions of equal width are called interference fringes.
