Wave Optics

In a biprism experiment, monochromatic light of wavelength '\(\lambda\)' is used. The distance between two coherent sources '\(\mathrm{d}\)' is kept constant. If the distance between slit and eyepiece '\(\mathrm{D}\)' is varied as \(D_1, D_2, D_3 \& D_4\) and corresponding measured fringe widths are \(Z_1, Z_2, Z_3\) and \(Z_4\) then