To determine the wavelength of the source, we need to know the fringe width. For this, the convex lens is removed and the experimental arrangement is as shown in fig. Now we move the eyepiece, so that fringes with good contrast are visible on eyepiece. The fringe width is measured by setting the cross wire of eyepiece on every fifth bright fringe and recording the position using micrometer screw. The average fringe width () is then determined by averaging the distance between every 5 fringes and then by dividing by 5. Knowing , d, D, can be determined.

To determine the thickness of transparent thin sheet (mica), the monochromatic source is replaced by white light source. In this case the fringe pattern consists of several fringe patterns corresponding to all wavelength and the resultant of this is a central white fringe surrounded by dark region.

The position of this central white fringe is recorded by focusing the cross wire of eye piece on it and taking this reading of micrometer scale. Now mica sheet is introduced in the path of one wave. (such that it blocks the one half of biprism). By doing it the one wave traverse an extra optical path and the path difference between the two waves is not same and entire fringe pattern shifts. The central white fringe is now shifted to another position of cross wire. If ‘S' is the shift in position of white fringe and be the refractive index of mica sheet, thickness ‘t' of mica sheet is given by

                        cm