1 The phenomenon of interference is used in testing the plainness of the surface of glass plate .

2 It two surfaces OA and OB are perfectly plane, the air film between them would gradually vary in thickness from O to A. The fringes are of equal thickness as each fringe is the locus of points at which the thickness of the film has a constant value.

3 To test the optical flatness of a surface the specimen surface to be tested(OB) is placed over an optically plane surface(OA)

4 The fringes are observed in the field of view.

5 If the fringes are of equal thickness then surface is plane.

6 If the fringes are not of equal thickness then surface is not plane.

7 In this way interference in wedge shaped film is used to test optical flatness of given glass plate.