To determine the separation between the two virtual sources ( d ), a convex lens of short focal length is introduced between the biprism and the eye piece, keeping the distance between the slit and eyepiece to be more than four times the focal length of lens. The lens is moved along the length of bench to a position where two images of slits are seen in the plane of cross wires of eye piece. The distance between these two images of slit is measured by setting the vertical cross wire successively on each of images and recording the two positions of cross wire using micrometer. Let this separation be d₁ . Now the lens is moved such that for another position of lens, again two images of slit are seen on eye piece. Let d₂ be the separation between these two images. Since these two positions of lens are conjugate, the separation between the virtual source ‘d ' is given as

                                                   

                             
                       Fig.11.1.3:Determination of 'd' in Fresnel Biprism experiment