S represents a pin hole illuminated by sunlight. The spherical wave front from S is incident on two pin holes S1 and S2 which are very close to each other and equidistant from S. Then the pin holes S1 and S2 act as two coherent sources of light of same intensity. The two sets of spherical wave fronts coming out of S1 and S2 interfere with each other in such a way as to produce a symmetrical pattern of varying intensity on the screen placed at a suitable distance D.