An adjustable slit is used to demonstrate the effect of slit width on diffraction patterns. As the width of the slit is varied, the diffraction pattern produced by a laser beam changes, with narrower slits producing wider patterns. This setup helps bridge the understanding between geometric optics, where light travels in straight lines, and wave optics, where light exhibits diffraction. The experiment highlights how the slit width and wavelength of light determine the diffraction pattern's characteristics.