Diffraction is the bending of waves around obstacles, such as openings in slits.
Diffraction will stretch out a light spot shown on a screen. Note, slit width has the same meaning as slit opening size. Use white paper taped to a wall as a screen to view the patterns. View patterns A,B,C and D on a screen. Be sure to keep the distance from the laser to the wall the same for each of the four patterns you view.