This device has been fully described in the first class dealing with interferences and called “The principle of interferences”. The reader is invited to use that lesson. We will just remind you that the system uses a primary source and two secondary sources that are either microscopic holes or thin slits (see figure 1) [1].
The wavefront emitted by the primary source is divided into two distinct secondary wavefronts by the holes or slits that act as secondary sources. As the holes or slits are quite narrow, the light diffracts towards the screen so we can observe the interferences between both diffraction figures of the secondary sources.