If you were to blow smoke into the space between the plane of  $S_1$  and  $S_2$  where $S_1$  and  $S_2$ are pinhole of standard YDSE and the viewing screen of figure, the smoke would show

 

The geometrical construction shown in figure is important for developing the mathematical description of interference. It is subject to misinterpretation, however, as it might suggest that the interference can only occur at the position of the screen. A better diagram for this situation is figure. Which shows paths of destructive and constructive interference all the way from the slits to the screen. These paths would be made visible by the smoke. Since fringes are present everywhere in space they are non–localized and their shapes are hyperboloid.