Light is incident on a single barrier with one narrow slit produces an interference patten due to the finite width of the slit. Every point on the wavefront behaves like a point light source and we observe an interference patten due to the interference of light from opposite sides of the slit.
This is the top diffraction pattern in the diagram.
When light is incident on a barrier with two parallel narrow slits, each slit behaves almost like a point source of light.
Each slit gives rise to almost circular wavefronts that spread out from the slit producing a diffraction pattern like the top pattern in the diagram.
The overall pattern produced is found by taking the single slit diffraction pattern as the envelope for the double slit diffraction patern, as if the single slit is split into two narrow slits.
