As an example of the interference phenomena, consider the formation of fringes in the arrangement shown in Figure 4.4. A transparent block A rests on another block B and the spacing d between them varies with position. Surface 2 is partly silvered and surface 3 is fully silvered and these surfaces are exposed to light at normal incidence. The interference pattern arising from the superposition of rays a (leaving A after reflection) and and b (leaving B after reflection) is shown in Figure 4.4. The pattern see in Figure 4.4 is an interference pattern of wedge fringes since they originate from a wedge-shaped alI gap.
If the incident light has a phase angle then and

Here, the phase difference arising from the block A can be ignored because its contribution is common to both rays and . Hence

When the phase difference between the two light beams is and the first line of the interference pattern is a dark line rising from destructive interference. As we go from the to fringe changes from and so

The factor is related to the fact that ray traverses the distance twice. The above method permits measurement of starting from the first dark fringe where .

Fringes will follow lines of constant and hence represent contour lines on an uneven surface. This method can be used to measure or the flatness of a surface, the latter being related to the straightness of the wedge fringes (Figure 4.4).