Two monochromatic coherent point sources S₁ and S₂ are separated by a distance L. Each source emits light of wavelength $\mathrm{\lambda }$; where L >> $\mathrm{\lambda }$. The line S₁S₂ when extended meets a screen perpendicular to it at point A. Then

If the screen is perpendicular to y-axis (line joining the sources), i.e., xz plane, the fringes will be circular (i.e., concentric circles with their centers on the point of intersection of the screen with y-axis). In this situation, central fringe will be bright if S₁, ${\mathrm{S}}_2=\mathrm{n\lambda }$ and dark if ${\mathrm{S}}_1{\mathrm{S}}_2=(2\mathrm{n}-1)\mathrm{\lambda }/2$. From all this, it is clear that shape of fringes depends on the nature of sources and direction of observation, i.e., position of screen.