A ripple tank is placed above a mirror and a projection screen. A horizontal bar, whose frequency can be varied, taps the surface of the water and produces plane waves. The waves are incident on a barrier containing an adjustable opening.

With a narrow single slit opening we see a plane wave emerge as a spherical wave on the other side of the barrier. This can be explained by the Huygens-Fresnel principle which states that each point in the slit forms its own spherical wavefront source. With a wider slit, more point sources arise, causing interference patterns due to the larger angles involved with a broader spatial distribution of these sources.